Adding upstream version 5.10.209.upstream/5.10.209upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

9 files changed, 6586 insertions, 0 deletions

diff --git a/kernel/printk/Makefile b/kernel/printk/Makefile
new file mode 100644
index 000000000..eee3dc9b6
--- /dev/null
+++ b/kernel/printk/Makefile
@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0-only
+obj-y	= printk.o
+obj-$(CONFIG_PRINTK)	+= printk_safe.o
+obj-$(CONFIG_A11Y_BRAILLE_CONSOLE)	+= braille.o
+obj-$(CONFIG_PRINTK)	+= printk_ringbuffer.o
diff --git a/kernel/printk/braille.c b/kernel/printk/braille.c
new file mode 100644
index 000000000..17a9591e5
--- /dev/null
+++ b/kernel/printk/braille.c
@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: GPL-2.0
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/kernel.h>
+#include <linux/console.h>
+#include <linux/errno.h>
+#include <linux/string.h>
+
+#include "console_cmdline.h"
+#include "braille.h"
+
+int _braille_console_setup(char **str, char **brl_options)
+{
+	size_t len;
+
+	len = str_has_prefix(*str, "brl,");
+	if (len) {
+		*brl_options = "";
+		*str += len;
+		return 0;
+	}
+
+	len = str_has_prefix(*str, "brl=");
+	if (len) {
+		*brl_options = *str + len;
+		*str = strchr(*brl_options, ',');
+		if (!*str) {
+			pr_err("need port name after brl=\n");
+			return -EINVAL;
+		}
+		*((*str)++) = 0;
+	}
+
+	return 0;
+}
+
+int
+_braille_register_console(struct console *console, struct console_cmdline *c)
+{
+	int rtn = 0;
+
+	if (c->brl_options) {
+		console->flags |= CON_BRL;
+		rtn = braille_register_console(console, c->index, c->options,
+					       c->brl_options);
+	}
+
+	return rtn;
+}
+
+int
+_braille_unregister_console(struct console *console)
+{
+	if (console->flags & CON_BRL)
+		return braille_unregister_console(console);
+
+	return 0;
+}
diff --git a/kernel/printk/braille.h b/kernel/printk/braille.h
new file mode 100644
index 000000000..123154f86
--- /dev/null
+++ b/kernel/printk/braille.h
@@ -0,0 +1,56 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _PRINTK_BRAILLE_H
+#define _PRINTK_BRAILLE_H
+
+#ifdef CONFIG_A11Y_BRAILLE_CONSOLE
+
+static inline void
+braille_set_options(struct console_cmdline *c, char *brl_options)
+{
+	c->brl_options = brl_options;
+}
+
+/*
+ * Setup console according to braille options.
+ * Return -EINVAL on syntax error, 0 on success (or no braille option was
+ * actually given).
+ * Modifies str to point to the serial options
+ * Sets brl_options to the parsed braille options.
+ */
+int
+_braille_console_setup(char **str, char **brl_options);
+
+int
+_braille_register_console(struct console *console, struct console_cmdline *c);
+
+int
+_braille_unregister_console(struct console *console);
+
+#else
+
+static inline void
+braille_set_options(struct console_cmdline *c, char *brl_options)
+{
+}
+
+static inline int
+_braille_console_setup(char **str, char **brl_options)
+{
+	return 0;
+}
+
+static inline int
+_braille_register_console(struct console *console, struct console_cmdline *c)
+{
+	return 0;
+}
+
+static inline int
+_braille_unregister_console(struct console *console)
+{
+	return 0;
+}
+
+#endif
+
+#endif
diff --git a/kernel/printk/console_cmdline.h b/kernel/printk/console_cmdline.h
new file mode 100644
index 000000000..3ca74ad39
--- /dev/null
+++ b/kernel/printk/console_cmdline.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _CONSOLE_CMDLINE_H
+#define _CONSOLE_CMDLINE_H
+
+struct console_cmdline
+{
+	char	name[16];			/* Name of the driver	    */
+	int	index;				/* Minor dev. to use	    */
+	bool	user_specified;			/* Specified by command line vs. platform */
+	char	*options;			/* Options for the driver   */
+#ifdef CONFIG_A11Y_BRAILLE_CONSOLE
+	char	*brl_options;			/* Options for braille driver */
+#endif
+};
+
+#endif
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
new file mode 100644
index 000000000..3a8fd4917
--- /dev/null
+++ b/kernel/printk/internal.h
@@ -0,0 +1,74 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * internal.h - printk internal definitions
+ */
+#include <linux/percpu.h>
+
+#ifdef CONFIG_PRINTK
+
+#define PRINTK_SAFE_CONTEXT_MASK	0x007ffffff
+#define PRINTK_NMI_DIRECT_CONTEXT_MASK	0x008000000
+#define PRINTK_NMI_CONTEXT_MASK		0xff0000000
+
+#define PRINTK_NMI_CONTEXT_OFFSET	0x010000000
+
+extern raw_spinlock_t logbuf_lock;
+
+__printf(4, 0)
+int vprintk_store(int facility, int level,
+		  const struct dev_printk_info *dev_info,
+		  const char *fmt, va_list args);
+
+__printf(1, 0) int vprintk_default(const char *fmt, va_list args);
+__printf(1, 0) int vprintk_deferred(const char *fmt, va_list args);
+__printf(1, 0) int vprintk_func(const char *fmt, va_list args);
+void __printk_safe_enter(void);
+void __printk_safe_exit(void);
+
+void printk_safe_init(void);
+bool printk_percpu_data_ready(void);
+
+#define printk_safe_enter_irqsave(flags)	\
+	do {					\
+		local_irq_save(flags);		\
+		__printk_safe_enter();		\
+	} while (0)
+
+#define printk_safe_exit_irqrestore(flags)	\
+	do {					\
+		__printk_safe_exit();		\
+		local_irq_restore(flags);	\
+	} while (0)
+
+#define printk_safe_enter_irq()		\
+	do {					\
+		local_irq_disable();		\
+		__printk_safe_enter();		\
+	} while (0)
+
+#define printk_safe_exit_irq()			\
+	do {					\
+		__printk_safe_exit();		\
+		local_irq_enable();		\
+	} while (0)
+
+void defer_console_output(void);
+
+#else
+
+__printf(1, 0) int vprintk_func(const char *fmt, va_list args) { return 0; }
+
+/*
+ * In !PRINTK builds we still export logbuf_lock spin_lock, console_sem
+ * semaphore and some of console functions (console_unlock()/etc.), so
+ * printk-safe must preserve the existing local IRQ guarantees.
+ */
+#define printk_safe_enter_irqsave(flags) local_irq_save(flags)
+#define printk_safe_exit_irqrestore(flags) local_irq_restore(flags)
+
+#define printk_safe_enter_irq() local_irq_disable()
+#define printk_safe_exit_irq() local_irq_enable()
+
+static inline void printk_safe_init(void) { }
+static inline bool printk_percpu_data_ready(void) { return false; }
+#endif /* CONFIG_PRINTK */
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
new file mode 100644
index 000000000..17a310dcb
--- /dev/null
+++ b/kernel/printk/printk.c
@@ -0,0 +1,3489 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ *  linux/kernel/printk.c
+ *
+ *  Copyright (C) 1991, 1992  Linus Torvalds
+ *
+ * Modified to make sys_syslog() more flexible: added commands to
+ * return the last 4k of kernel messages, regardless of whether
+ * they've been read or not.  Added option to suppress kernel printk's
+ * to the console.  Added hook for sending the console messages
+ * elsewhere, in preparation for a serial line console (someday).
+ * Ted Ts'o, 2/11/93.
+ * Modified for sysctl support, 1/8/97, Chris Horn.
+ * Fixed SMP synchronization, 08/08/99, Manfred Spraul
+ *     manfred@colorfullife.com
+ * Rewrote bits to get rid of console_lock
+ *	01Mar01 Andrew Morton
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/tty.h>
+#include <linux/tty_driver.h>
+#include <linux/console.h>
+#include <linux/init.h>
+#include <linux/jiffies.h>
+#include <linux/nmi.h>
+#include <linux/module.h>
+#include <linux/moduleparam.h>
+#include <linux/delay.h>
+#include <linux/smp.h>
+#include <linux/security.h>
+#include <linux/memblock.h>
+#include <linux/syscalls.h>
+#include <linux/crash_core.h>
+#include <linux/ratelimit.h>
+#include <linux/kmsg_dump.h>
+#include <linux/syslog.h>
+#include <linux/cpu.h>
+#include <linux/rculist.h>
+#include <linux/poll.h>
+#include <linux/irq_work.h>
+#include <linux/ctype.h>
+#include <linux/uio.h>
+#include <linux/sched/clock.h>
+#include <linux/sched/debug.h>
+#include <linux/sched/task_stack.h>
+
+#include <linux/uaccess.h>
+#include <asm/sections.h>
+
+#include <trace/events/initcall.h>
+#define CREATE_TRACE_POINTS
+#include <trace/events/printk.h>
+
+#include "printk_ringbuffer.h"
+#include "console_cmdline.h"
+#include "braille.h"
+#include "internal.h"
+
+int console_printk[4] = {
+	CONSOLE_LOGLEVEL_DEFAULT,	/* console_loglevel */
+	MESSAGE_LOGLEVEL_DEFAULT,	/* default_message_loglevel */
+	CONSOLE_LOGLEVEL_MIN,		/* minimum_console_loglevel */
+	CONSOLE_LOGLEVEL_DEFAULT,	/* default_console_loglevel */
+};
+EXPORT_SYMBOL_GPL(console_printk);
+
+atomic_t ignore_console_lock_warning __read_mostly = ATOMIC_INIT(0);
+EXPORT_SYMBOL(ignore_console_lock_warning);
+
+/*
+ * Low level drivers may need that to know if they can schedule in
+ * their unblank() callback or not. So let's export it.
+ */
+int oops_in_progress;
+EXPORT_SYMBOL(oops_in_progress);
+
+/*
+ * console_sem protects the console_drivers list, and also
+ * provides serialisation for access to the entire console
+ * driver system.
+ */
+static DEFINE_SEMAPHORE(console_sem);
+struct console *console_drivers;
+EXPORT_SYMBOL_GPL(console_drivers);
+
+/*
+ * System may need to suppress printk message under certain
+ * circumstances, like after kernel panic happens.
+ */
+int __read_mostly suppress_printk;
+
+#ifdef CONFIG_LOCKDEP
+static struct lockdep_map console_lock_dep_map = {
+	.name = "console_lock"
+};
+#endif
+
+enum devkmsg_log_bits {
+	__DEVKMSG_LOG_BIT_ON = 0,
+	__DEVKMSG_LOG_BIT_OFF,
+	__DEVKMSG_LOG_BIT_LOCK,
+};
+
+enum devkmsg_log_masks {
+	DEVKMSG_LOG_MASK_ON             = BIT(__DEVKMSG_LOG_BIT_ON),
+	DEVKMSG_LOG_MASK_OFF            = BIT(__DEVKMSG_LOG_BIT_OFF),
+	DEVKMSG_LOG_MASK_LOCK           = BIT(__DEVKMSG_LOG_BIT_LOCK),
+};
+
+/* Keep both the 'on' and 'off' bits clear, i.e. ratelimit by default: */
+#define DEVKMSG_LOG_MASK_DEFAULT	0
+
+static unsigned int __read_mostly devkmsg_log = DEVKMSG_LOG_MASK_DEFAULT;
+
+static int __control_devkmsg(char *str)
+{
+	size_t len;
+
+	if (!str)
+		return -EINVAL;
+
+	len = str_has_prefix(str, "on");
+	if (len) {
+		devkmsg_log = DEVKMSG_LOG_MASK_ON;
+		return len;
+	}
+
+	len = str_has_prefix(str, "off");
+	if (len) {
+		devkmsg_log = DEVKMSG_LOG_MASK_OFF;
+		return len;
+	}
+
+	len = str_has_prefix(str, "ratelimit");
+	if (len) {
+		devkmsg_log = DEVKMSG_LOG_MASK_DEFAULT;
+		return len;
+	}
+
+	return -EINVAL;
+}
+
+static int __init control_devkmsg(char *str)
+{
+	if (__control_devkmsg(str) < 0) {
+		pr_warn("printk.devkmsg: bad option string '%s'\n", str);
+		return 1;
+	}
+
+	/*
+	 * Set sysctl string accordingly:
+	 */
+	if (devkmsg_log == DEVKMSG_LOG_MASK_ON)
+		strcpy(devkmsg_log_str, "on");
+	else if (devkmsg_log == DEVKMSG_LOG_MASK_OFF)
+		strcpy(devkmsg_log_str, "off");
+	/* else "ratelimit" which is set by default. */
+
+	/*
+	 * Sysctl cannot change it anymore. The kernel command line setting of
+	 * this parameter is to force the setting to be permanent throughout the
+	 * runtime of the system. This is a precation measure against userspace
+	 * trying to be a smarta** and attempting to change it up on us.
+	 */
+	devkmsg_log |= DEVKMSG_LOG_MASK_LOCK;
+
+	return 1;
+}
+__setup("printk.devkmsg=", control_devkmsg);
+
+char devkmsg_log_str[DEVKMSG_STR_MAX_SIZE] = "ratelimit";
+
+int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
+			      void *buffer, size_t *lenp, loff_t *ppos)
+{
+	char old_str[DEVKMSG_STR_MAX_SIZE];
+	unsigned int old;
+	int err;
+
+	if (write) {
+		if (devkmsg_log & DEVKMSG_LOG_MASK_LOCK)
+			return -EINVAL;
+
+		old = devkmsg_log;
+		strncpy(old_str, devkmsg_log_str, DEVKMSG_STR_MAX_SIZE);
+	}
+
+	err = proc_dostring(table, write, buffer, lenp, ppos);
+	if (err)
+		return err;
+
+	if (write) {
+		err = __control_devkmsg(devkmsg_log_str);
+
+		/*
+		 * Do not accept an unknown string OR a known string with
+		 * trailing crap...
+		 */
+		if (err < 0 || (err + 1 != *lenp)) {
+
+			/* ... and restore old setting. */
+			devkmsg_log = old;
+			strncpy(devkmsg_log_str, old_str, DEVKMSG_STR_MAX_SIZE);
+
+			return -EINVAL;
+		}
+	}
+
+	return 0;
+}
+
+/* Number of registered extended console drivers. */
+static int nr_ext_console_drivers;
+
+/*
+ * Helper macros to handle lockdep when locking/unlocking console_sem. We use
+ * macros instead of functions so that _RET_IP_ contains useful information.
+ */
+#define down_console_sem() do { \
+	down(&console_sem);\
+	mutex_acquire(&console_lock_dep_map, 0, 0, _RET_IP_);\
+} while (0)
+
+static int __down_trylock_console_sem(unsigned long ip)
+{
+	int lock_failed;
+	unsigned long flags;
+
+	/*
+	 * Here and in __up_console_sem() we need to be in safe mode,
+	 * because spindump/WARN/etc from under console ->lock will
+	 * deadlock in printk()->down_trylock_console_sem() otherwise.
+	 */
+	printk_safe_enter_irqsave(flags);
+	lock_failed = down_trylock(&console_sem);
+	printk_safe_exit_irqrestore(flags);
+
+	if (lock_failed)
+		return 1;
+	mutex_acquire(&console_lock_dep_map, 0, 1, ip);
+	return 0;
+}
+#define down_trylock_console_sem() __down_trylock_console_sem(_RET_IP_)
+
+static void __up_console_sem(unsigned long ip)
+{
+	unsigned long flags;
+
+	mutex_release(&console_lock_dep_map, ip);
+
+	printk_safe_enter_irqsave(flags);
+	up(&console_sem);
+	printk_safe_exit_irqrestore(flags);
+}
+#define up_console_sem() __up_console_sem(_RET_IP_)
+
+/*
+ * This is used for debugging the mess that is the VT code by
+ * keeping track if we have the console semaphore held. It's
+ * definitely not the perfect debug tool (we don't know if _WE_
+ * hold it and are racing, but it helps tracking those weird code
+ * paths in the console code where we end up in places I want
+ * locked without the console sempahore held).
+ */
+static int console_locked, console_suspended;
+
+/*
+ * If exclusive_console is non-NULL then only this console is to be printed to.
+ */
+static struct console *exclusive_console;
+
+/*
+ *	Array of consoles built from command line options (console=)
+ */
+
+#define MAX_CMDLINECONSOLES 8
+
+static struct console_cmdline console_cmdline[MAX_CMDLINECONSOLES];
+
+static int preferred_console = -1;
+static bool has_preferred_console;
+int console_set_on_cmdline;
+EXPORT_SYMBOL(console_set_on_cmdline);
+
+/* Flag: console code may call schedule() */
+static int console_may_schedule;
+
+enum con_msg_format_flags {
+	MSG_FORMAT_DEFAULT	= 0,
+	MSG_FORMAT_SYSLOG	= (1 << 0),
+};
+
+static int console_msg_format = MSG_FORMAT_DEFAULT;
+
+/*
+ * The printk log buffer consists of a sequenced collection of records, each
+ * containing variable length message text. Every record also contains its
+ * own meta-data (@info).
+ *
+ * Every record meta-data carries the timestamp in microseconds, as well as
+ * the standard userspace syslog level and syslog facility. The usual kernel
+ * messages use LOG_KERN; userspace-injected messages always carry a matching
+ * syslog facility, by default LOG_USER. The origin of every message can be
+ * reliably determined that way.
+ *
+ * The human readable log message of a record is available in @text, the
+ * length of the message text in @text_len. The stored message is not
+ * terminated.
+ *
+ * Optionally, a record can carry a dictionary of properties (key/value
+ * pairs), to provide userspace with a machine-readable message context.
+ *
+ * Examples for well-defined, commonly used property names are:
+ *   DEVICE=b12:8               device identifier
+ *                                b12:8         block dev_t
+ *                                c127:3        char dev_t
+ *                                n8            netdev ifindex
+ *                                +sound:card0  subsystem:devname
+ *   SUBSYSTEM=pci              driver-core subsystem name
+ *
+ * Valid characters in property names are [a-zA-Z0-9.-_]. Property names
+ * and values are terminated by a '\0' character.
+ *
+ * Example of record values:
+ *   record.text_buf                = "it's a line" (unterminated)
+ *   record.info.seq                = 56
+ *   record.info.ts_nsec            = 36863
+ *   record.info.text_len           = 11
+ *   record.info.facility           = 0 (LOG_KERN)
+ *   record.info.flags              = 0
+ *   record.info.level              = 3 (LOG_ERR)
+ *   record.info.caller_id          = 299 (task 299)
+ *   record.info.dev_info.subsystem = "pci" (terminated)
+ *   record.info.dev_info.device    = "+pci:0000:00:01.0" (terminated)
+ *
+ * The 'struct printk_info' buffer must never be directly exported to
+ * userspace, it is a kernel-private implementation detail that might
+ * need to be changed in the future, when the requirements change.
+ *
+ * /dev/kmsg exports the structured data in the following line format:
+ *   "<level>,<sequnum>,<timestamp>,<contflag>[,additional_values, ... ];<message text>\n"
+ *
+ * Users of the export format should ignore possible additional values
+ * separated by ',', and find the message after the ';' character.
+ *
+ * The optional key/value pairs are attached as continuation lines starting
+ * with a space character and terminated by a newline. All possible
+ * non-prinatable characters are escaped in the "\xff" notation.
+ */
+
+enum log_flags {
+	LOG_NEWLINE	= 2,	/* text ended with a newline */
+	LOG_CONT	= 8,	/* text is a fragment of a continuation line */
+};
+
+/*
+ * The logbuf_lock protects kmsg buffer, indices, counters.  This can be taken
+ * within the scheduler's rq lock. It must be released before calling
+ * console_unlock() or anything else that might wake up a process.
+ */
+DEFINE_RAW_SPINLOCK(logbuf_lock);
+
+/*
+ * Helper macros to lock/unlock logbuf_lock and switch between
+ * printk-safe/unsafe modes.
+ */
+#define logbuf_lock_irq()				\
+	do {						\
+		printk_safe_enter_irq();		\
+		raw_spin_lock(&logbuf_lock);		\
+	} while (0)
+
+#define logbuf_unlock_irq()				\
+	do {						\
+		raw_spin_unlock(&logbuf_lock);		\
+		printk_safe_exit_irq();			\
+	} while (0)
+
+#define logbuf_lock_irqsave(flags)			\
+	do {						\
+		printk_safe_enter_irqsave(flags);	\
+		raw_spin_lock(&logbuf_lock);		\
+	} while (0)
+
+#define logbuf_unlock_irqrestore(flags)		\
+	do {						\
+		raw_spin_unlock(&logbuf_lock);		\
+		printk_safe_exit_irqrestore(flags);	\
+	} while (0)
+
+#ifdef CONFIG_PRINTK
+DECLARE_WAIT_QUEUE_HEAD(log_wait);
+/* the next printk record to read by syslog(READ) or /proc/kmsg */
+static u64 syslog_seq;
+static size_t syslog_partial;
+static bool syslog_time;
+
+/* the next printk record to write to the console */
+static u64 console_seq;
+static u64 exclusive_console_stop_seq;
+static unsigned long console_dropped;
+
+/* the next printk record to read after the last 'clear' command */
+static u64 clear_seq;
+
+#ifdef CONFIG_PRINTK_CALLER
+#define PREFIX_MAX		48
+#else
+#define PREFIX_MAX		32
+#endif
+#define LOG_LINE_MAX		(1024 - PREFIX_MAX)
+
+#define LOG_LEVEL(v)		((v) & 0x07)
+#define LOG_FACILITY(v)		((v) >> 3 & 0xff)
+
+/* record buffer */
+#define LOG_ALIGN __alignof__(unsigned long)
+#define __LOG_BUF_LEN (1 << CONFIG_LOG_BUF_SHIFT)
+#define LOG_BUF_LEN_MAX (u32)(1 << 31)
+static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
+static char *log_buf = __log_buf;
+static u32 log_buf_len = __LOG_BUF_LEN;
+
+/*
+ * Define the average message size. This only affects the number of
+ * descriptors that will be available. Underestimating is better than
+ * overestimating (too many available descriptors is better than not enough).
+ */
+#define PRB_AVGBITS 5	/* 32 character average length */
+
+#if CONFIG_LOG_BUF_SHIFT <= PRB_AVGBITS
+#error CONFIG_LOG_BUF_SHIFT value too small.
+#endif
+_DEFINE_PRINTKRB(printk_rb_static, CONFIG_LOG_BUF_SHIFT - PRB_AVGBITS,
+		 PRB_AVGBITS, &__log_buf[0]);
+
+static struct printk_ringbuffer printk_rb_dynamic;
+
+static struct printk_ringbuffer *prb = &printk_rb_static;
+
+/*
+ * We cannot access per-CPU data (e.g. per-CPU flush irq_work) before
+ * per_cpu_areas are initialised. This variable is set to true when
+ * it's safe to access per-CPU data.
+ */
+static bool __printk_percpu_data_ready __read_mostly;
+
+bool printk_percpu_data_ready(void)
+{
+	return __printk_percpu_data_ready;
+}
+
+/* Return log buffer address */
+char *log_buf_addr_get(void)
+{
+	return log_buf;
+}
+
+/* Return log buffer size */
+u32 log_buf_len_get(void)
+{
+	return log_buf_len;
+}
+
+/*
+ * Define how much of the log buffer we could take at maximum. The value
+ * must be greater than two. Note that only half of the buffer is available
+ * when the index points to the middle.
+ */
+#define MAX_LOG_TAKE_PART 4
+static const char trunc_msg[] = "<truncated>";
+
+static void truncate_msg(u16 *text_len, u16 *trunc_msg_len)
+{
+	/*
+	 * The message should not take the whole buffer. Otherwise, it might
+	 * get removed too soon.
+	 */
+	u32 max_text_len = log_buf_len / MAX_LOG_TAKE_PART;
+
+	if (*text_len > max_text_len)
+		*text_len = max_text_len;
+
+	/* enable the warning message (if there is room) */
+	*trunc_msg_len = strlen(trunc_msg);
+	if (*text_len >= *trunc_msg_len)
+		*text_len -= *trunc_msg_len;
+	else
+		*trunc_msg_len = 0;
+}
+
+/* insert record into the buffer, discard old ones, update heads */
+static int log_store(u32 caller_id, int facility, int level,
+		     enum log_flags flags, u64 ts_nsec,
+		     const struct dev_printk_info *dev_info,
+		     const char *text, u16 text_len)
+{
+	struct prb_reserved_entry e;
+	struct printk_record r;
+	u16 trunc_msg_len = 0;
+
+	prb_rec_init_wr(&r, text_len);
+
+	if (!prb_reserve(&e, prb, &r)) {
+		/* truncate the message if it is too long for empty buffer */
+		truncate_msg(&text_len, &trunc_msg_len);
+		prb_rec_init_wr(&r, text_len + trunc_msg_len);
+		/* survive when the log buffer is too small for trunc_msg */
+		if (!prb_reserve(&e, prb, &r))
+			return 0;
+	}
+
+	/* fill message */
+	memcpy(&r.text_buf[0], text, text_len);
+	if (trunc_msg_len)
+		memcpy(&r.text_buf[text_len], trunc_msg, trunc_msg_len);
+	r.info->text_len = text_len + trunc_msg_len;
+	r.info->facility = facility;
+	r.info->level = level & 7;
+	r.info->flags = flags & 0x1f;
+	if (ts_nsec > 0)
+		r.info->ts_nsec = ts_nsec;
+	else
+		r.info->ts_nsec = local_clock();
+	r.info->caller_id = caller_id;
+	if (dev_info)
+		memcpy(&r.info->dev_info, dev_info, sizeof(r.info->dev_info));
+
+	/* A message without a trailing newline can be continued. */
+	if (!(flags & LOG_NEWLINE))
+		prb_commit(&e);
+	else
+		prb_final_commit(&e);
+
+	return (text_len + trunc_msg_len);
+}
+
+int dmesg_restrict = IS_ENABLED(CONFIG_SECURITY_DMESG_RESTRICT);
+
+static int syslog_action_restricted(int type)
+{
+	if (dmesg_restrict)
+		return 1;
+	/*
+	 * Unless restricted, we allow "read all" and "get buffer size"
+	 * for everybody.
+	 */
+	return type != SYSLOG_ACTION_READ_ALL &&
+	       type != SYSLOG_ACTION_SIZE_BUFFER;
+}
+
+static int check_syslog_permissions(int type, int source)
+{
+	/*
+	 * If this is from /proc/kmsg and we've already opened it, then we've
+	 * already done the capabilities checks at open time.
+	 */
+	if (source == SYSLOG_FROM_PROC && type != SYSLOG_ACTION_OPEN)
+		goto ok;
+
+	if (syslog_action_restricted(type)) {
+		if (capable(CAP_SYSLOG))
+			goto ok;
+		/*
+		 * For historical reasons, accept CAP_SYS_ADMIN too, with
+		 * a warning.
+		 */
+		if (capable(CAP_SYS_ADMIN)) {
+			pr_warn_once("%s (%d): Attempt to access syslog with "
+				     "CAP_SYS_ADMIN but no CAP_SYSLOG "
+				     "(deprecated).\n",
+				 current->comm, task_pid_nr(current));
+			goto ok;
+		}
+		return -EPERM;
+	}
+ok:
+	return security_syslog(type);
+}
+
+static void append_char(char **pp, char *e, char c)
+{
+	if (*pp < e)
+		*(*pp)++ = c;
+}
+
+static ssize_t info_print_ext_header(char *buf, size_t size,
+				     struct printk_info *info)
+{
+	u64 ts_usec = info->ts_nsec;
+	char caller[20];
+#ifdef CONFIG_PRINTK_CALLER
+	u32 id = info->caller_id;
+
+	snprintf(caller, sizeof(caller), ",caller=%c%u",
+		 id & 0x80000000 ? 'C' : 'T', id & ~0x80000000);
+#else
+	caller[0] = '\0';
+#endif
+
+	do_div(ts_usec, 1000);
+
+	return scnprintf(buf, size, "%u,%llu,%llu,%c%s;",
+			 (info->facility << 3) | info->level, info->seq,
+			 ts_usec, info->flags & LOG_CONT ? 'c' : '-', caller);
+}
+
+static ssize_t msg_add_ext_text(char *buf, size_t size,
+				const char *text, size_t text_len,
+				unsigned char endc)
+{
+	char *p = buf, *e = buf + size;
+	size_t i;
+
+	/* escape non-printable characters */
+	for (i = 0; i < text_len; i++) {
+		unsigned char c = text[i];
+
+		if (c < ' ' || c >= 127 || c == '\\')
+			p += scnprintf(p, e - p, "\\x%02x", c);
+		else
+			append_char(&p, e, c);
+	}
+	append_char(&p, e, endc);
+
+	return p - buf;
+}
+
+static ssize_t msg_add_dict_text(char *buf, size_t size,
+				 const char *key, const char *val)
+{
+	size_t val_len = strlen(val);
+	ssize_t len;
+
+	if (!val_len)
+		return 0;
+
+	len = msg_add_ext_text(buf, size, "", 0, ' ');	/* dict prefix */
+	len += msg_add_ext_text(buf + len, size - len, key, strlen(key), '=');
+	len += msg_add_ext_text(buf + len, size - len, val, val_len, '\n');
+
+	return len;
+}
+
+static ssize_t msg_print_ext_body(char *buf, size_t size,
+				  char *text, size_t text_len,
+				  struct dev_printk_info *dev_info)
+{
+	ssize_t len;
+
+	len = msg_add_ext_text(buf, size, text, text_len, '\n');
+
+	if (!dev_info)
+		goto out;
+
+	len += msg_add_dict_text(buf + len, size - len, "SUBSYSTEM",
+				 dev_info->subsystem);
+	len += msg_add_dict_text(buf + len, size - len, "DEVICE",
+				 dev_info->device);
+out:
+	return len;
+}
+
+/* /dev/kmsg - userspace message inject/listen interface */
+struct devkmsg_user {
+	u64 seq;
+	struct ratelimit_state rs;
+	struct mutex lock;
+	char buf[CONSOLE_EXT_LOG_MAX];
+
+	struct printk_info info;
+	char text_buf[CONSOLE_EXT_LOG_MAX];
+	struct printk_record record;
+};
+
+static __printf(3, 4) __cold
+int devkmsg_emit(int facility, int level, const char *fmt, ...)
+{
+	va_list args;
+	int r;
+
+	va_start(args, fmt);
+	r = vprintk_emit(facility, level, NULL, fmt, args);
+	va_end(args);
+
+	return r;
+}
+
+static ssize_t devkmsg_write(struct kiocb *iocb, struct iov_iter *from)
+{
+	char *buf, *line;
+	int level = default_message_loglevel;
+	int facility = 1;	/* LOG_USER */
+	struct file *file = iocb->ki_filp;
+	struct devkmsg_user *user = file->private_data;
+	size_t len = iov_iter_count(from);
+	ssize_t ret = len;
+
+	if (!user || len > LOG_LINE_MAX)
+		return -EINVAL;
+
+	/* Ignore when user logging is disabled. */
+	if (devkmsg_log & DEVKMSG_LOG_MASK_OFF)
+		return len;
+
+	/* Ratelimit when not explicitly enabled. */
+	if (!(devkmsg_log & DEVKMSG_LOG_MASK_ON)) {
+		if (!___ratelimit(&user->rs, current->comm))
+			return ret;
+	}
+
+	buf = kmalloc(len+1, GFP_KERNEL);
+	if (buf == NULL)
+		return -ENOMEM;
+
+	buf[len] = '\0';
+	if (!copy_from_iter_full(buf, len, from)) {
+		kfree(buf);
+		return -EFAULT;
+	}
+
+	/*
+	 * Extract and skip the syslog prefix <[0-9]*>. Coming from userspace
+	 * the decimal value represents 32bit, the lower 3 bit are the log
+	 * level, the rest are the log facility.
+	 *
+	 * If no prefix or no userspace facility is specified, we
+	 * enforce LOG_USER, to be able to reliably distinguish
+	 * kernel-generated messages from userspace-injected ones.
+	 */
+	line = buf;
+	if (line[0] == '<') {
+		char *endp = NULL;
+		unsigned int u;
+
+		u = simple_strtoul(line + 1, &endp, 10);
+		if (endp && endp[0] == '>') {
+			level = LOG_LEVEL(u);
+			if (LOG_FACILITY(u) != 0)
+				facility = LOG_FACILITY(u);
+			endp++;
+			len -= endp - line;
+			line = endp;
+		}
+	}
+
+	devkmsg_emit(facility, level, "%s", line);
+	kfree(buf);
+	return ret;
+}
+
+static ssize_t devkmsg_read(struct file *file, char __user *buf,
+			    size_t count, loff_t *ppos)
+{
+	struct devkmsg_user *user = file->private_data;
+	struct printk_record *r = &user->record;
+	size_t len;
+	ssize_t ret;
+
+	if (!user)
+		return -EBADF;
+
+	ret = mutex_lock_interruptible(&user->lock);
+	if (ret)
+		return ret;
+
+	logbuf_lock_irq();
+	if (!prb_read_valid(prb, user->seq, r)) {
+		if (file->f_flags & O_NONBLOCK) {
+			ret = -EAGAIN;
+			logbuf_unlock_irq();
+			goto out;
+		}
+
+		logbuf_unlock_irq();
+		ret = wait_event_interruptible(log_wait,
+					prb_read_valid(prb, user->seq, r));
+		if (ret)
+			goto out;
+		logbuf_lock_irq();
+	}
+
+	if (r->info->seq != user->seq) {
+		/* our last seen message is gone, return error and reset */
+		user->seq = r->info->seq;
+		ret = -EPIPE;
+		logbuf_unlock_irq();
+		goto out;
+	}
+
+	len = info_print_ext_header(user->buf, sizeof(user->buf), r->info);
+	len += msg_print_ext_body(user->buf + len, sizeof(user->buf) - len,
+				  &r->text_buf[0], r->info->text_len,
+				  &r->info->dev_info);
+
+	user->seq = r->info->seq + 1;
+	logbuf_unlock_irq();
+
+	if (len > count) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	if (copy_to_user(buf, user->buf, len)) {
+		ret = -EFAULT;
+		goto out;
+	}
+	ret = len;
+out:
+	mutex_unlock(&user->lock);
+	return ret;
+}
+
+/*
+ * Be careful when modifying this function!!!
+ *
+ * Only few operations are supported because the device works only with the
+ * entire variable length messages (records). Non-standard values are
+ * returned in the other cases and has been this way for quite some time.
+ * User space applications might depend on this behavior.
+ */
+static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
+{
+	struct devkmsg_user *user = file->private_data;
+	loff_t ret = 0;
+
+	if (!user)
+		return -EBADF;
+	if (offset)
+		return -ESPIPE;
+
+	logbuf_lock_irq();
+	switch (whence) {
+	case SEEK_SET:
+		/* the first record */
+		user->seq = prb_first_valid_seq(prb);
+		break;
+	case SEEK_DATA:
+		/*
+		 * The first record after the last SYSLOG_ACTION_CLEAR,
+		 * like issued by 'dmesg -c'. Reading /dev/kmsg itself
+		 * changes no global state, and does not clear anything.
+		 */
+		user->seq = clear_seq;
+		break;
+	case SEEK_END:
+		/* after the last record */
+		user->seq = prb_next_seq(prb);
+		break;
+	default:
+		ret = -EINVAL;
+	}
+	logbuf_unlock_irq();
+	return ret;
+}
+
+static __poll_t devkmsg_poll(struct file *file, poll_table *wait)
+{
+	struct devkmsg_user *user = file->private_data;
+	struct printk_info info;
+	__poll_t ret = 0;
+
+	if (!user)
+		return EPOLLERR|EPOLLNVAL;
+
+	poll_wait(file, &log_wait, wait);
+
+	logbuf_lock_irq();
+	if (prb_read_valid_info(prb, user->seq, &info, NULL)) {
+		/* return error when data has vanished underneath us */
+		if (info.seq != user->seq)
+			ret = EPOLLIN|EPOLLRDNORM|EPOLLERR|EPOLLPRI;
+		else
+			ret = EPOLLIN|EPOLLRDNORM;
+	}
+	logbuf_unlock_irq();
+
+	return ret;
+}
+
+static int devkmsg_open(struct inode *inode, struct file *file)
+{
+	struct devkmsg_user *user;
+	int err;
+
+	if (devkmsg_log & DEVKMSG_LOG_MASK_OFF)
+		return -EPERM;
+
+	/* write-only does not need any file context */
+	if ((file->f_flags & O_ACCMODE) != O_WRONLY) {
+		err = check_syslog_permissions(SYSLOG_ACTION_READ_ALL,
+					       SYSLOG_FROM_READER);
+		if (err)
+			return err;
+	}
+
+	user = kmalloc(sizeof(struct devkmsg_user), GFP_KERNEL);
+	if (!user)
+		return -ENOMEM;
+
+	ratelimit_default_init(&user->rs);
+	ratelimit_set_flags(&user->rs, RATELIMIT_MSG_ON_RELEASE);
+
+	mutex_init(&user->lock);
+
+	prb_rec_init_rd(&user->record, &user->info,
+			&user->text_buf[0], sizeof(user->text_buf));
+
+	logbuf_lock_irq();
+	user->seq = prb_first_valid_seq(prb);
+	logbuf_unlock_irq();
+
+	file->private_data = user;
+	return 0;
+}
+
+static int devkmsg_release(struct inode *inode, struct file *file)
+{
+	struct devkmsg_user *user = file->private_data;
+
+	if (!user)
+		return 0;
+
+	ratelimit_state_exit(&user->rs);
+
+	mutex_destroy(&user->lock);
+	kfree(user);
+	return 0;
+}
+
+const struct file_operations kmsg_fops = {
+	.open = devkmsg_open,
+	.read = devkmsg_read,
+	.write_iter = devkmsg_write,
+	.llseek = devkmsg_llseek,
+	.poll = devkmsg_poll,
+	.release = devkmsg_release,
+};
+
+#ifdef CONFIG_CRASH_CORE
+/*
+ * This appends the listed symbols to /proc/vmcore
+ *
+ * /proc/vmcore is used by various utilities, like crash and makedumpfile to
+ * obtain access to symbols that are otherwise very difficult to locate.  These
+ * symbols are specifically used so that utilities can access and extract the
+ * dmesg log from a vmcore file after a crash.
+ */
+void log_buf_vmcoreinfo_setup(void)
+{
+	struct dev_printk_info *dev_info = NULL;
+
+	VMCOREINFO_SYMBOL(prb);
+	VMCOREINFO_SYMBOL(printk_rb_static);
+	VMCOREINFO_SYMBOL(clear_seq);
+
+	/*
+	 * Export struct size and field offsets. User space tools can
+	 * parse it and detect any changes to structure down the line.
+	 */
+
+	VMCOREINFO_STRUCT_SIZE(printk_ringbuffer);
+	VMCOREINFO_OFFSET(printk_ringbuffer, desc_ring);
+	VMCOREINFO_OFFSET(printk_ringbuffer, text_data_ring);
+	VMCOREINFO_OFFSET(printk_ringbuffer, fail);
+
+	VMCOREINFO_STRUCT_SIZE(prb_desc_ring);
+	VMCOREINFO_OFFSET(prb_desc_ring, count_bits);
+	VMCOREINFO_OFFSET(prb_desc_ring, descs);
+	VMCOREINFO_OFFSET(prb_desc_ring, infos);
+	VMCOREINFO_OFFSET(prb_desc_ring, head_id);
+	VMCOREINFO_OFFSET(prb_desc_ring, tail_id);
+
+	VMCOREINFO_STRUCT_SIZE(prb_desc);
+	VMCOREINFO_OFFSET(prb_desc, state_var);
+	VMCOREINFO_OFFSET(prb_desc, text_blk_lpos);
+
+	VMCOREINFO_STRUCT_SIZE(prb_data_blk_lpos);
+	VMCOREINFO_OFFSET(prb_data_blk_lpos, begin);
+	VMCOREINFO_OFFSET(prb_data_blk_lpos, next);
+
+	VMCOREINFO_STRUCT_SIZE(printk_info);
+	VMCOREINFO_OFFSET(printk_info, seq);
+	VMCOREINFO_OFFSET(printk_info, ts_nsec);
+	VMCOREINFO_OFFSET(printk_info, text_len);
+	VMCOREINFO_OFFSET(printk_info, caller_id);
+	VMCOREINFO_OFFSET(printk_info, dev_info);
+
+	VMCOREINFO_STRUCT_SIZE(dev_printk_info);
+	VMCOREINFO_OFFSET(dev_printk_info, subsystem);
+	VMCOREINFO_LENGTH(printk_info_subsystem, sizeof(dev_info->subsystem));
+	VMCOREINFO_OFFSET(dev_printk_info, device);
+	VMCOREINFO_LENGTH(printk_info_device, sizeof(dev_info->device));
+
+	VMCOREINFO_STRUCT_SIZE(prb_data_ring);
+	VMCOREINFO_OFFSET(prb_data_ring, size_bits);
+	VMCOREINFO_OFFSET(prb_data_ring, data);
+	VMCOREINFO_OFFSET(prb_data_ring, head_lpos);
+	VMCOREINFO_OFFSET(prb_data_ring, tail_lpos);
+
+	VMCOREINFO_SIZE(atomic_long_t);
+	VMCOREINFO_TYPE_OFFSET(atomic_long_t, counter);
+}
+#endif
+
+/* requested log_buf_len from kernel cmdline */
+static unsigned long __initdata new_log_buf_len;
+
+/* we practice scaling the ring buffer by powers of 2 */
+static void __init log_buf_len_update(u64 size)
+{
+	if (size > (u64)LOG_BUF_LEN_MAX) {
+		size = (u64)LOG_BUF_LEN_MAX;
+		pr_err("log_buf over 2G is not supported.\n");
+	}
+
+	if (size)
+		size = roundup_pow_of_two(size);
+	if (size > log_buf_len)
+		new_log_buf_len = (unsigned long)size;
+}
+
+/* save requested log_buf_len since it's too early to process it */
+static int __init log_buf_len_setup(char *str)
+{
+	u64 size;
+
+	if (!str)
+		return -EINVAL;
+
+	size = memparse(str, &str);
+
+	log_buf_len_update(size);
+
+	return 0;
+}
+early_param("log_buf_len", log_buf_len_setup);
+
+#ifdef CONFIG_SMP
+#define __LOG_CPU_MAX_BUF_LEN (1 << CONFIG_LOG_CPU_MAX_BUF_SHIFT)
+
+static void __init log_buf_add_cpu(void)
+{
+	unsigned int cpu_extra;
+
+	/*
+	 * archs should set up cpu_possible_bits properly with
+	 * set_cpu_possible() after setup_arch() but just in
+	 * case lets ensure this is valid.
+	 */
+	if (num_possible_cpus() == 1)
+		return;
+
+	cpu_extra = (num_possible_cpus() - 1) * __LOG_CPU_MAX_BUF_LEN;
+
+	/* by default this will only continue through for large > 64 CPUs */
+	if (cpu_extra <= __LOG_BUF_LEN / 2)
+		return;
+
+	pr_info("log_buf_len individual max cpu contribution: %d bytes\n",
+		__LOG_CPU_MAX_BUF_LEN);
+	pr_info("log_buf_len total cpu_extra contributions: %d bytes\n",
+		cpu_extra);
+	pr_info("log_buf_len min size: %d bytes\n", __LOG_BUF_LEN);
+
+	log_buf_len_update(cpu_extra + __LOG_BUF_LEN);
+}
+#else /* !CONFIG_SMP */
+static inline void log_buf_add_cpu(void) {}
+#endif /* CONFIG_SMP */
+
+static void __init set_percpu_data_ready(void)
+{
+	printk_safe_init();
+	/* Make sure we set this flag only after printk_safe() init is done */
+	barrier();
+	__printk_percpu_data_ready = true;
+}
+
+static unsigned int __init add_to_rb(struct printk_ringbuffer *rb,
+				     struct printk_record *r)
+{
+	struct prb_reserved_entry e;
+	struct printk_record dest_r;
+
+	prb_rec_init_wr(&dest_r, r->info->text_len);
+
+	if (!prb_reserve(&e, rb, &dest_r))
+		return 0;
+
+	memcpy(&dest_r.text_buf[0], &r->text_buf[0], r->info->text_len);
+	dest_r.info->text_len = r->info->text_len;
+	dest_r.info->facility = r->info->facility;
+	dest_r.info->level = r->info->level;
+	dest_r.info->flags = r->info->flags;
+	dest_r.info->ts_nsec = r->info->ts_nsec;
+	dest_r.info->caller_id = r->info->caller_id;
+	memcpy(&dest_r.info->dev_info, &r->info->dev_info, sizeof(dest_r.info->dev_info));
+
+	prb_final_commit(&e);
+
+	return prb_record_text_space(&e);
+}
+
+static char setup_text_buf[LOG_LINE_MAX] __initdata;
+
+void __init setup_log_buf(int early)
+{
+	struct printk_info *new_infos;
+	unsigned int new_descs_count;
+	struct prb_desc *new_descs;
+	struct printk_info info;
+	struct printk_record r;
+	size_t new_descs_size;
+	size_t new_infos_size;
+	unsigned long flags;
+	char *new_log_buf;
+	unsigned int free;
+	u64 seq;
+
+	/*
+	 * Some archs call setup_log_buf() multiple times - first is very
+	 * early, e.g. from setup_arch(), and second - when percpu_areas
+	 * are initialised.
+	 */
+	if (!early)
+		set_percpu_data_ready();
+
+	if (log_buf != __log_buf)
+		return;
+
+	if (!early && !new_log_buf_len)
+		log_buf_add_cpu();
+
+	if (!new_log_buf_len)
+		return;
+
+	new_descs_count = new_log_buf_len >> PRB_AVGBITS;
+	if (new_descs_count == 0) {
+		pr_err("new_log_buf_len: %lu too small\n", new_log_buf_len);
+		return;
+	}
+
+	new_log_buf = memblock_alloc(new_log_buf_len, LOG_ALIGN);
+	if (unlikely(!new_log_buf)) {
+		pr_err("log_buf_len: %lu text bytes not available\n",
+		       new_log_buf_len);
+		return;
+	}
+
+	new_descs_size = new_descs_count * sizeof(struct prb_desc);
+	new_descs = memblock_alloc(new_descs_size, LOG_ALIGN);
+	if (unlikely(!new_descs)) {
+		pr_err("log_buf_len: %zu desc bytes not available\n",
+		       new_descs_size);
+		goto err_free_log_buf;
+	}
+
+	new_infos_size = new_descs_count * sizeof(struct printk_info);
+	new_infos = memblock_alloc(new_infos_size, LOG_ALIGN);
+	if (unlikely(!new_infos)) {
+		pr_err("log_buf_len: %zu info bytes not available\n",
+		       new_infos_size);
+		goto err_free_descs;
+	}
+
+	prb_rec_init_rd(&r, &info, &setup_text_buf[0], sizeof(setup_text_buf));
+
+	prb_init(&printk_rb_dynamic,
+		 new_log_buf, ilog2(new_log_buf_len),
+		 new_descs, ilog2(new_descs_count),
+		 new_infos);
+
+	logbuf_lock_irqsave(flags);
+
+	log_buf_len = new_log_buf_len;
+	log_buf = new_log_buf;
+	new_log_buf_len = 0;
+
+	free = __LOG_BUF_LEN;
+	prb_for_each_record(0, &printk_rb_static, seq, &r)
+		free -= add_to_rb(&printk_rb_dynamic, &r);
+
+	/*
+	 * This is early enough that everything is still running on the
+	 * boot CPU and interrupts are disabled. So no new messages will
+	 * appear during the transition to the dynamic buffer.
+	 */
+	prb = &printk_rb_dynamic;
+
+	logbuf_unlock_irqrestore(flags);
+
+	if (seq != prb_next_seq(&printk_rb_static)) {
+		pr_err("dropped %llu messages\n",
+		       prb_next_seq(&printk_rb_static) - seq);
+	}
+
+	pr_info("log_buf_len: %u bytes\n", log_buf_len);
+	pr_info("early log buf free: %u(%u%%)\n",
+		free, (free * 100) / __LOG_BUF_LEN);
+	return;
+
+err_free_descs:
+	memblock_free(__pa(new_descs), new_descs_size);
+err_free_log_buf:
+	memblock_free(__pa(new_log_buf), new_log_buf_len);
+}
+
+static bool __read_mostly ignore_loglevel;
+
+static int __init ignore_loglevel_setup(char *str)
+{
+	ignore_loglevel = true;
+	pr_info("debug: ignoring loglevel setting.\n");
+
+	return 0;
+}
+
+early_param("ignore_loglevel", ignore_loglevel_setup);
+module_param(ignore_loglevel, bool, S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(ignore_loglevel,
+		 "ignore loglevel setting (prints all kernel messages to the console)");
+
+static bool suppress_message_printing(int level)
+{
+	return (level >= console_loglevel && !ignore_loglevel);
+}
+
+#ifdef CONFIG_BOOT_PRINTK_DELAY
+
+static int boot_delay; /* msecs delay after each printk during bootup */
+static unsigned long long loops_per_msec;	/* based on boot_delay */
+
+static int __init boot_delay_setup(char *str)
+{
+	unsigned long lpj;
+
+	lpj = preset_lpj ? preset_lpj : 1000000;	/* some guess */
+	loops_per_msec = (unsigned long long)lpj / 1000 * HZ;
+
+	get_option(&str, &boot_delay);
+	if (boot_delay > 10 * 1000)
+		boot_delay = 0;
+
+	pr_debug("boot_delay: %u, preset_lpj: %ld, lpj: %lu, "
+		"HZ: %d, loops_per_msec: %llu\n",
+		boot_delay, preset_lpj, lpj, HZ, loops_per_msec);
+	return 0;
+}
+early_param("boot_delay", boot_delay_setup);
+
+static void boot_delay_msec(int level)
+{
+	unsigned long long k;
+	unsigned long timeout;
+
+	if ((boot_delay == 0 || system_state >= SYSTEM_RUNNING)
+		|| suppress_message_printing(level)) {
+		return;
+	}
+
+	k = (unsigned long long)loops_per_msec * boot_delay;
+
+	timeout = jiffies + msecs_to_jiffies(boot_delay);
+	while (k) {
+		k--;
+		cpu_relax();
+		/*
+		 * use (volatile) jiffies to prevent
+		 * compiler reduction; loop termination via jiffies
+		 * is secondary and may or may not happen.
+		 */
+		if (time_after(jiffies, timeout))
+			break;
+		touch_nmi_watchdog();
+	}
+}
+#else
+static inline void boot_delay_msec(int level)
+{
+}
+#endif
+
+static bool printk_time = IS_ENABLED(CONFIG_PRINTK_TIME);
+module_param_named(time, printk_time, bool, S_IRUGO | S_IWUSR);
+
+static size_t print_syslog(unsigned int level, char *buf)
+{
+	return sprintf(buf, "<%u>", level);
+}
+
+static size_t print_time(u64 ts, char *buf)
+{
+	unsigned long rem_nsec = do_div(ts, 1000000000);
+
+	return sprintf(buf, "[%5lu.%06lu]",
+		       (unsigned long)ts, rem_nsec / 1000);
+}
+
+#ifdef CONFIG_PRINTK_CALLER
+static size_t print_caller(u32 id, char *buf)
+{
+	char caller[12];
+
+	snprintf(caller, sizeof(caller), "%c%u",
+		 id & 0x80000000 ? 'C' : 'T', id & ~0x80000000);
+	return sprintf(buf, "[%6s]", caller);
+}
+#else
+#define print_caller(id, buf) 0
+#endif
+
+static size_t info_print_prefix(const struct printk_info  *info, bool syslog,
+				bool time, char *buf)
+{
+	size_t len = 0;
+
+	if (syslog)
+		len = print_syslog((info->facility << 3) | info->level, buf);
+
+	if (time)
+		len += print_time(info->ts_nsec, buf + len);
+
+	len += print_caller(info->caller_id, buf + len);
+
+	if (IS_ENABLED(CONFIG_PRINTK_CALLER) || time) {
+		buf[len++] = ' ';
+		buf[len] = '\0';
+	}
+
+	return len;
+}
+
+/*
+ * Prepare the record for printing. The text is shifted within the given
+ * buffer to avoid a need for another one. The following operations are
+ * done:
+ *
+ *   - Add prefix for each line.
+ *   - Drop truncated lines that no longer fit into the buffer.
+ *   - Add the trailing newline that has been removed in vprintk_store().
+ *   - Add a string terminator.
+ *
+ * Since the produced string is always terminated, the maximum possible
+ * return value is @r->text_buf_size - 1;
+ *
+ * Return: The length of the updated/prepared text, including the added
+ * prefixes and the newline. The terminator is not counted. The dropped
+ * line(s) are not counted.
+ */
+static size_t record_print_text(struct printk_record *r, bool syslog,
+				bool time)
+{
+	size_t text_len = r->info->text_len;
+	size_t buf_size = r->text_buf_size;
+	char *text = r->text_buf;
+	char prefix[PREFIX_MAX];
+	bool truncated = false;
+	size_t prefix_len;
+	size_t line_len;
+	size_t len = 0;
+	char *next;
+
+	/*
+	 * If the message was truncated because the buffer was not large
+	 * enough, treat the available text as if it were the full text.
+	 */
+	if (text_len > buf_size)
+		text_len = buf_size;
+
+	prefix_len = info_print_prefix(r->info, syslog, time, prefix);
+
+	/*
+	 * @text_len: bytes of unprocessed text
+	 * @line_len: bytes of current line _without_ newline
+	 * @text:     pointer to beginning of current line
+	 * @len:      number of bytes prepared in r->text_buf
+	 */
+	for (;;) {
+		next = memchr(text, '\n', text_len);
+		if (next) {
+			line_len = next - text;
+		} else {
+			/* Drop truncated line(s). */
+			if (truncated)
+				break;
+			line_len = text_len;
+		}
+
+		/*
+		 * Truncate the text if there is not enough space to add the
+		 * prefix and a trailing newline and a terminator.
+		 */
+		if (len + prefix_len + text_len + 1 + 1 > buf_size) {
+			/* Drop even the current line if no space. */
+			if (len + prefix_len + line_len + 1 + 1 > buf_size)
+				break;
+
+			text_len = buf_size - len - prefix_len - 1 - 1;
+			truncated = true;
+		}
+
+		memmove(text + prefix_len, text, text_len);
+		memcpy(text, prefix, prefix_len);
+
+		/*
+		 * Increment the prepared length to include the text and
+		 * prefix that were just moved+copied. Also increment for the
+		 * newline at the end of this line. If this is the last line,
+		 * there is no newline, but it will be added immediately below.
+		 */
+		len += prefix_len + line_len + 1;
+		if (text_len == line_len) {
+			/*
+			 * This is the last line. Add the trailing newline
+			 * removed in vprintk_store().
+			 */
+			text[prefix_len + line_len] = '\n';
+			break;
+		}
+
+		/*
+		 * Advance beyond the added prefix and the related line with
+		 * its newline.
+		 */
+		text += prefix_len + line_len + 1;
+
+		/*
+		 * The remaining text has only decreased by the line with its
+		 * newline.
+		 *
+		 * Note that @text_len can become zero. It happens when @text
+		 * ended with a newline (either due to truncation or the
+		 * original string ending with "\n\n"). The loop is correctly
+		 * repeated and (if not truncated) an empty line with a prefix
+		 * will be prepared.
+		 */
+		text_len -= line_len + 1;
+	}
+
+	/*
+	 * If a buffer was provided, it will be terminated. Space for the
+	 * string terminator is guaranteed to be available. The terminator is
+	 * not counted in the return value.
+	 */
+	if (buf_size > 0)
+		r->text_buf[len] = 0;
+
+	return len;
+}
+
+static size_t get_record_print_text_size(struct printk_info *info,
+					 unsigned int line_count,
+					 bool syslog, bool time)
+{
+	char prefix[PREFIX_MAX];
+	size_t prefix_len;
+
+	prefix_len = info_print_prefix(info, syslog, time, prefix);
+
+	/*
+	 * Each line will be preceded with a prefix. The intermediate
+	 * newlines are already within the text, but a final trailing
+	 * newline will be added.
+	 */
+	return ((prefix_len * line_count) + info->text_len + 1);
+}
+
+static int syslog_print(char __user *buf, int size)
+{
+	struct printk_info info;
+	struct printk_record r;
+	char *text;
+	int len = 0;
+
+	text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL);
+	if (!text)
+		return -ENOMEM;
+
+	prb_rec_init_rd(&r, &info, text, LOG_LINE_MAX + PREFIX_MAX);
+
+	while (size > 0) {
+		size_t n;
+		size_t skip;
+
+		logbuf_lock_irq();
+		if (!prb_read_valid(prb, syslog_seq, &r)) {
+			logbuf_unlock_irq();
+			break;
+		}
+		if (r.info->seq != syslog_seq) {
+			/* message is gone, move to next valid one */
+			syslog_seq = r.info->seq;
+			syslog_partial = 0;
+		}
+
+		/*
+		 * To keep reading/counting partial line consistent,
+		 * use printk_time value as of the beginning of a line.
+		 */
+		if (!syslog_partial)
+			syslog_time = printk_time;
+
+		skip = syslog_partial;
+		n = record_print_text(&r, true, syslog_time);
+		if (n - syslog_partial <= size) {
+			/* message fits into buffer, move forward */
+			syslog_seq = r.info->seq + 1;
+			n -= syslog_partial;
+			syslog_partial = 0;
+		} else if (!len){
+			/* partial read(), remember position */
+			n = size;
+			syslog_partial += n;
+		} else
+			n = 0;
+		logbuf_unlock_irq();
+
+		if (!n)
+			break;
+
+		if (copy_to_user(buf, text + skip, n)) {
+			if (!len)
+				len = -EFAULT;
+			break;
+		}
+
+		len += n;
+		size -= n;
+		buf += n;
+	}
+
+	kfree(text);
+	return len;
+}
+
+static int syslog_print_all(char __user *buf, int size, bool clear)
+{
+	struct printk_info info;
+	unsigned int line_count;
+	struct printk_record r;
+	char *text;
+	int len = 0;
+	u64 seq;
+	bool time;
+
+	text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL);
+	if (!text)
+		return -ENOMEM;
+
+	time = printk_time;
+	logbuf_lock_irq();
+	/*
+	 * Find first record that fits, including all following records,
+	 * into the user-provided buffer for this dump.
+	 */
+	prb_for_each_info(clear_seq, prb, seq, &info, &line_count)
+		len += get_record_print_text_size(&info, line_count, true, time);
+
+	/* move first record forward until length fits into the buffer */
+	prb_for_each_info(clear_seq, prb, seq, &info, &line_count) {
+		if (len <= size)
+			break;
+		len -= get_record_print_text_size(&info, line_count, true, time);
+	}
+
+	prb_rec_init_rd(&r, &info, text, LOG_LINE_MAX + PREFIX_MAX);
+
+	len = 0;
+	prb_for_each_record(seq, prb, seq, &r) {
+		int textlen;
+
+		textlen = record_print_text(&r, true, time);
+
+		if (len + textlen > size) {
+			seq--;
+			break;
+		}
+
+		logbuf_unlock_irq();
+		if (copy_to_user(buf + len, text, textlen))
+			len = -EFAULT;
+		else
+			len += textlen;
+		logbuf_lock_irq();
+
+		if (len < 0)
+			break;
+	}
+
+	if (clear)
+		clear_seq = seq;
+	logbuf_unlock_irq();
+
+	kfree(text);
+	return len;
+}
+
+static void syslog_clear(void)
+{
+	logbuf_lock_irq();
+	clear_seq = prb_next_seq(prb);
+	logbuf_unlock_irq();
+}
+
+int do_syslog(int type, char __user *buf, int len, int source)
+{
+	struct printk_info info;
+	bool clear = false;
+	static int saved_console_loglevel = LOGLEVEL_DEFAULT;
+	int error;
+
+	error = check_syslog_permissions(type, source);
+	if (error)
+		return error;
+
+	switch (type) {
+	case SYSLOG_ACTION_CLOSE:	/* Close log */
+		break;
+	case SYSLOG_ACTION_OPEN:	/* Open log */
+		break;
+	case SYSLOG_ACTION_READ:	/* Read from log */
+		if (!buf || len < 0)
+			return -EINVAL;
+		if (!len)
+			return 0;
+		if (!access_ok(buf, len))
+			return -EFAULT;
+		error = wait_event_interruptible(log_wait,
+				prb_read_valid(prb, syslog_seq, NULL));
+		if (error)
+			return error;
+		error = syslog_print(buf, len);
+		break;
+	/* Read/clear last kernel messages */
+	case SYSLOG_ACTION_READ_CLEAR:
+		clear = true;
+		fallthrough;
+	/* Read last kernel messages */
+	case SYSLOG_ACTION_READ_ALL:
+		if (!buf || len < 0)
+			return -EINVAL;
+		if (!len)
+			return 0;
+		if (!access_ok(buf, len))
+			return -EFAULT;
+		error = syslog_print_all(buf, len, clear);
+		break;
+	/* Clear ring buffer */
+	case SYSLOG_ACTION_CLEAR:
+		syslog_clear();
+		break;
+	/* Disable logging to console */
+	case SYSLOG_ACTION_CONSOLE_OFF:
+		if (saved_console_loglevel == LOGLEVEL_DEFAULT)
+			saved_console_loglevel = console_loglevel;
+		console_loglevel = minimum_console_loglevel;
+		break;
+	/* Enable logging to console */
+	case SYSLOG_ACTION_CONSOLE_ON:
+		if (saved_console_loglevel != LOGLEVEL_DEFAULT) {
+			console_loglevel = saved_console_loglevel;
+			saved_console_loglevel = LOGLEVEL_DEFAULT;
+		}
+		break;
+	/* Set level of messages printed to console */
+	case SYSLOG_ACTION_CONSOLE_LEVEL:
+		if (len < 1 || len > 8)
+			return -EINVAL;
+		if (len < minimum_console_loglevel)
+			len = minimum_console_loglevel;
+		console_loglevel = len;
+		/* Implicitly re-enable logging to console */
+		saved_console_loglevel = LOGLEVEL_DEFAULT;
+		break;
+	/* Number of chars in the log buffer */
+	case SYSLOG_ACTION_SIZE_UNREAD:
+		logbuf_lock_irq();
+		if (!prb_read_valid_info(prb, syslog_seq, &info, NULL)) {
+			/* No unread messages. */
+			logbuf_unlock_irq();
+			return 0;
+		}
+		if (info.seq != syslog_seq) {
+			/* messages are gone, move to first one */
+			syslog_seq = info.seq;
+			syslog_partial = 0;
+		}
+		if (source == SYSLOG_FROM_PROC) {
+			/*
+			 * Short-cut for poll(/"proc/kmsg") which simply checks
+			 * for pending data, not the size; return the count of
+			 * records, not the length.
+			 */
+			error = prb_next_seq(prb) - syslog_seq;
+		} else {
+			bool time = syslog_partial ? syslog_time : printk_time;
+			unsigned int line_count;
+			u64 seq;
+
+			prb_for_each_info(syslog_seq, prb, seq, &info,
+					  &line_count) {
+				error += get_record_print_text_size(&info, line_count,
+								    true, time);
+				time = printk_time;
+			}
+			error -= syslog_partial;
+		}
+		logbuf_unlock_irq();
+		break;
+	/* Size of the log buffer */
+	case SYSLOG_ACTION_SIZE_BUFFER:
+		error = log_buf_len;
+		break;
+	default:
+		error = -EINVAL;
+		break;
+	}
+
+	return error;
+}
+
+SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
+{
+	return do_syslog(type, buf, len, SYSLOG_FROM_READER);
+}
+
+/*
+ * Special console_lock variants that help to reduce the risk of soft-lockups.
+ * They allow to pass console_lock to another printk() call using a busy wait.
+ */
+
+#ifdef CONFIG_LOCKDEP
+static struct lockdep_map console_owner_dep_map = {
+	.name = "console_owner"
+};
+#endif
+
+static DEFINE_RAW_SPINLOCK(console_owner_lock);
+static struct task_struct *console_owner;
+static bool console_waiter;
+
+/**
+ * console_lock_spinning_enable - mark beginning of code where another
+ *	thread might safely busy wait
+ *
+ * This basically converts console_lock into a spinlock. This marks
+ * the section where the console_lock owner can not sleep, because
+ * there may be a waiter spinning (like a spinlock). Also it must be
+ * ready to hand over the lock at the end of the section.
+ */
+static void console_lock_spinning_enable(void)
+{
+	raw_spin_lock(&console_owner_lock);
+	console_owner = current;
+	raw_spin_unlock(&console_owner_lock);
+
+	/* The waiter may spin on us after setting console_owner */
+	spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
+}
+
+/**
+ * console_lock_spinning_disable_and_check - mark end of code where another
+ *	thread was able to busy wait and check if there is a waiter
+ *
+ * This is called at the end of the section where spinning is allowed.
+ * It has two functions. First, it is a signal that it is no longer
+ * safe to start busy waiting for the lock. Second, it checks if
+ * there is a busy waiter and passes the lock rights to her.
+ *
+ * Important: Callers lose the lock if there was a busy waiter.
+ *	They must not touch items synchronized by console_lock
+ *	in this case.
+ *
+ * Return: 1 if the lock rights were passed, 0 otherwise.
+ */
+static int console_lock_spinning_disable_and_check(void)
+{
+	int waiter;
+
+	raw_spin_lock(&console_owner_lock);
+	waiter = READ_ONCE(console_waiter);
+	console_owner = NULL;
+	raw_spin_unlock(&console_owner_lock);
+
+	if (!waiter) {
+		spin_release(&console_owner_dep_map, _THIS_IP_);
+		return 0;
+	}
+
+	/* The waiter is now free to continue */
+	WRITE_ONCE(console_waiter, false);
+
+	spin_release(&console_owner_dep_map, _THIS_IP_);
+
+	/*
+	 * Hand off console_lock to waiter. The waiter will perform
+	 * the up(). After this, the waiter is the console_lock owner.
+	 */
+	mutex_release(&console_lock_dep_map, _THIS_IP_);
+	return 1;
+}
+
+/**
+ * console_trylock_spinning - try to get console_lock by busy waiting
+ *
+ * This allows to busy wait for the console_lock when the current
+ * owner is running in specially marked sections. It means that
+ * the current owner is running and cannot reschedule until it
+ * is ready to lose the lock.
+ *
+ * Return: 1 if we got the lock, 0 othrewise
+ */
+static int console_trylock_spinning(void)
+{
+	struct task_struct *owner = NULL;
+	bool waiter;
+	bool spin = false;
+	unsigned long flags;
+
+	if (console_trylock())
+		return 1;
+
+	printk_safe_enter_irqsave(flags);
+
+	raw_spin_lock(&console_owner_lock);
+	owner = READ_ONCE(console_owner);
+	waiter = READ_ONCE(console_waiter);
+	if (!waiter && owner && owner != current) {
+		WRITE_ONCE(console_waiter, true);
+		spin = true;
+	}
+	raw_spin_unlock(&console_owner_lock);
+
+	/*
+	 * If there is an active printk() writing to the
+	 * consoles, instead of having it write our data too,
+	 * see if we can offload that load from the active
+	 * printer, and do some printing ourselves.
+	 * Go into a spin only if there isn't already a waiter
+	 * spinning, and there is an active printer, and
+	 * that active printer isn't us (recursive printk?).
+	 */
+	if (!spin) {
+		printk_safe_exit_irqrestore(flags);
+		return 0;
+	}
+
+	/* We spin waiting for the owner to release us */
+	spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
+	/* Owner will clear console_waiter on hand off */
+	while (READ_ONCE(console_waiter))
+		cpu_relax();
+	spin_release(&console_owner_dep_map, _THIS_IP_);
+
+	printk_safe_exit_irqrestore(flags);
+	/*
+	 * The owner passed the console lock to us.
+	 * Since we did not spin on console lock, annotate
+	 * this as a trylock. Otherwise lockdep will
+	 * complain.
+	 */
+	mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);
+
+	return 1;
+}
+
+/*
+ * Call the console drivers, asking them to write out
+ * log_buf[start] to log_buf[end - 1].
+ * The console_lock must be held.
+ */
+static void call_console_drivers(const char *ext_text, size_t ext_len,
+				 const char *text, size_t len)
+{
+	static char dropped_text[64];
+	size_t dropped_len = 0;
+	struct console *con;
+
+	trace_console_rcuidle(text, len);
+
+	if (!console_drivers)
+		return;
+
+	if (console_dropped) {
+		dropped_len = snprintf(dropped_text, sizeof(dropped_text),
+				       "** %lu printk messages dropped **\n",
+				       console_dropped);
+		console_dropped = 0;
+	}
+
+	for_each_console(con) {
+		if (exclusive_console && con != exclusive_console)
+			continue;
+		if (!(con->flags & CON_ENABLED))
+			continue;
+		if (!con->write)
+			continue;
+		if (!cpu_online(smp_processor_id()) &&
+		    !(con->flags & CON_ANYTIME))
+			continue;
+		if (con->flags & CON_EXTENDED)
+			con->write(con, ext_text, ext_len);
+		else {
+			if (dropped_len)
+				con->write(con, dropped_text, dropped_len);
+			con->write(con, text, len);
+		}
+	}
+}
+
+int printk_delay_msec __read_mostly;
+
+static inline void printk_delay(void)
+{
+	if (unlikely(printk_delay_msec)) {
+		int m = printk_delay_msec;
+
+		while (m--) {
+			mdelay(1);
+			touch_nmi_watchdog();
+		}
+	}
+}
+
+static inline u32 printk_caller_id(void)
+{
+	return in_task() ? task_pid_nr(current) :
+		0x80000000 + raw_smp_processor_id();
+}
+
+static size_t log_output(int facility, int level, enum log_flags lflags,
+			 const struct dev_printk_info *dev_info,
+			 char *text, size_t text_len)
+{
+	const u32 caller_id = printk_caller_id();
+
+	if (lflags & LOG_CONT) {
+		struct prb_reserved_entry e;
+		struct printk_record r;
+
+		prb_rec_init_wr(&r, text_len);
+		if (prb_reserve_in_last(&e, prb, &r, caller_id, LOG_LINE_MAX)) {
+			memcpy(&r.text_buf[r.info->text_len], text, text_len);
+			r.info->text_len += text_len;
+			if (lflags & LOG_NEWLINE) {
+				r.info->flags |= LOG_NEWLINE;
+				prb_final_commit(&e);
+			} else {
+				prb_commit(&e);
+			}
+			return text_len;
+		}
+	}
+
+	/* Store it in the record log */
+	return log_store(caller_id, facility, level, lflags, 0,
+			 dev_info, text, text_len);
+}
+
+/* Must be called under logbuf_lock. */
+int vprintk_store(int facility, int level,
+		  const struct dev_printk_info *dev_info,
+		  const char *fmt, va_list args)
+{
+	static char textbuf[LOG_LINE_MAX];
+	char *text = textbuf;
+	size_t text_len;
+	enum log_flags lflags = 0;
+
+	/*
+	 * The printf needs to come first; we need the syslog
+	 * prefix which might be passed-in as a parameter.
+	 */
+	text_len = vscnprintf(text, sizeof(textbuf), fmt, args);
+
+	/* mark and strip a trailing newline */
+	if (text_len && text[text_len-1] == '\n') {
+		text_len--;
+		lflags |= LOG_NEWLINE;
+	}
+
+	/* strip kernel syslog prefix and extract log level or control flags */
+	if (facility == 0) {
+		int kern_level;
+
+		while ((kern_level = printk_get_level(text)) != 0) {
+			switch (kern_level) {
+			case '0' ... '7':
+				if (level == LOGLEVEL_DEFAULT)
+					level = kern_level - '0';
+				break;
+			case 'c':	/* KERN_CONT */
+				lflags |= LOG_CONT;
+			}
+
+			text_len -= 2;
+			text += 2;
+		}
+	}
+
+	if (level == LOGLEVEL_DEFAULT)
+		level = default_message_loglevel;
+
+	if (dev_info)
+		lflags |= LOG_NEWLINE;
+
+	return log_output(facility, level, lflags, dev_info, text, text_len);
+}
+
+asmlinkage int vprintk_emit(int facility, int level,
+			    const struct dev_printk_info *dev_info,
+			    const char *fmt, va_list args)
+{
+	int printed_len;
+	bool in_sched = false;
+	unsigned long flags;
+
+	/* Suppress unimportant messages after panic happens */
+	if (unlikely(suppress_printk))
+		return 0;
+
+	if (level == LOGLEVEL_SCHED) {
+		level = LOGLEVEL_DEFAULT;
+		in_sched = true;
+	}
+
+	boot_delay_msec(level);
+	printk_delay();
+
+	/* This stops the holder of console_sem just where we want him */
+	logbuf_lock_irqsave(flags);
+	printed_len = vprintk_store(facility, level, dev_info, fmt, args);
+	logbuf_unlock_irqrestore(flags);
+
+	/* If called from the scheduler, we can not call up(). */
+	if (!in_sched) {
+		/*
+		 * Disable preemption to avoid being preempted while holding
+		 * console_sem which would prevent anyone from printing to
+		 * console
+		 */
+		preempt_disable();
+		/*
+		 * Try to acquire and then immediately release the console
+		 * semaphore.  The release will print out buffers and wake up
+		 * /dev/kmsg and syslog() users.
+		 */
+		if (console_trylock_spinning())
+			console_unlock();
+		preempt_enable();
+	}
+
+	wake_up_klogd();
+	return printed_len;
+}
+EXPORT_SYMBOL(vprintk_emit);
+
+asmlinkage int vprintk(const char *fmt, va_list args)
+{
+	return vprintk_func(fmt, args);
+}
+EXPORT_SYMBOL(vprintk);
+
+int vprintk_default(const char *fmt, va_list args)
+{
+	return vprintk_emit(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
+}
+EXPORT_SYMBOL_GPL(vprintk_default);
+
+/**
+ * printk - print a kernel message
+ * @fmt: format string
+ *
+ * This is printk(). It can be called from any context. We want it to work.
+ *
+ * We try to grab the console_lock. If we succeed, it's easy - we log the
+ * output and call the console drivers.  If we fail to get the semaphore, we
+ * place the output into the log buffer and return. The current holder of
+ * the console_sem will notice the new output in console_unlock(); and will
+ * send it to the consoles before releasing the lock.
+ *
+ * One effect of this deferred printing is that code which calls printk() and
+ * then changes console_loglevel may break. This is because console_loglevel
+ * is inspected when the actual printing occurs.
+ *
+ * See also:
+ * printf(3)
+ *
+ * See the vsnprintf() documentation for format string extensions over C99.
+ */
+asmlinkage __visible int printk(const char *fmt, ...)
+{
+	va_list args;
+	int r;
+
+	va_start(args, fmt);
+	r = vprintk_func(fmt, args);
+	va_end(args);
+
+	return r;
+}
+EXPORT_SYMBOL(printk);
+
+#else /* CONFIG_PRINTK */
+
+#define LOG_LINE_MAX		0
+#define PREFIX_MAX		0
+#define printk_time		false
+
+#define prb_read_valid(rb, seq, r)	false
+#define prb_first_valid_seq(rb)		0
+
+static u64 syslog_seq;
+static u64 console_seq;
+static u64 exclusive_console_stop_seq;
+static unsigned long console_dropped;
+
+static size_t record_print_text(const struct printk_record *r,
+				bool syslog, bool time)
+{
+	return 0;
+}
+static ssize_t info_print_ext_header(char *buf, size_t size,
+				     struct printk_info *info)
+{
+	return 0;
+}
+static ssize_t msg_print_ext_body(char *buf, size_t size,
+				  char *text, size_t text_len,
+				  struct dev_printk_info *dev_info) { return 0; }
+static void console_lock_spinning_enable(void) { }
+static int console_lock_spinning_disable_and_check(void) { return 0; }
+static void call_console_drivers(const char *ext_text, size_t ext_len,
+				 const char *text, size_t len) {}
+static bool suppress_message_printing(int level) { return false; }
+
+#endif /* CONFIG_PRINTK */
+
+#ifdef CONFIG_EARLY_PRINTK
+struct console *early_console;
+
+asmlinkage __visible void early_printk(const char *fmt, ...)
+{
+	va_list ap;
+	char buf[512];
+	int n;
+
+	if (!early_console)
+		return;
+
+	va_start(ap, fmt);
+	n = vscnprintf(buf, sizeof(buf), fmt, ap);
+	va_end(ap);
+
+	early_console->write(early_console, buf, n);
+}
+#endif
+
+static int __add_preferred_console(char *name, int idx, char *options,
+				   char *brl_options, bool user_specified)
+{
+	struct console_cmdline *c;
+	int i;
+
+	/*
+	 *	See if this tty is not yet registered, and
+	 *	if we have a slot free.
+	 */
+	for (i = 0, c = console_cmdline;
+	     i < MAX_CMDLINECONSOLES && c->name[0];
+	     i++, c++) {
+		if (strcmp(c->name, name) == 0 && c->index == idx) {
+			if (!brl_options)
+				preferred_console = i;
+			if (user_specified)
+				c->user_specified = true;
+			return 0;
+		}
+	}
+	if (i == MAX_CMDLINECONSOLES)
+		return -E2BIG;
+	if (!brl_options)
+		preferred_console = i;
+	strlcpy(c->name, name, sizeof(c->name));
+	c->options = options;
+	c->user_specified = user_specified;
+	braille_set_options(c, brl_options);
+
+	c->index = idx;
+	return 0;
+}
+
+static int __init console_msg_format_setup(char *str)
+{
+	if (!strcmp(str, "syslog"))
+		console_msg_format = MSG_FORMAT_SYSLOG;
+	if (!strcmp(str, "default"))
+		console_msg_format = MSG_FORMAT_DEFAULT;
+	return 1;
+}
+__setup("console_msg_format=", console_msg_format_setup);
+
+/*
+ * Set up a console.  Called via do_early_param() in init/main.c
+ * for each "console=" parameter in the boot command line.
+ */
+static int __init console_setup(char *str)
+{
+	char buf[sizeof(console_cmdline[0].name) + 4]; /* 4 for "ttyS" */
+	char *s, *options, *brl_options = NULL;
+	int idx;
+
+	/*
+	 * console="" or console=null have been suggested as a way to
+	 * disable console output. Use ttynull that has been created
+	 * for exacly this purpose.
+	 */
+	if (str[0] == 0 || strcmp(str, "null") == 0) {
+		__add_preferred_console("ttynull", 0, NULL, NULL, true);
+		return 1;
+	}
+
+	if (_braille_console_setup(&str, &brl_options))
+		return 1;
+
+	/*
+	 * Decode str into name, index, options.
+	 */
+	if (str[0] >= '0' && str[0] <= '9') {
+		strcpy(buf, "ttyS");
+		strncpy(buf + 4, str, sizeof(buf) - 5);
+	} else {
+		strncpy(buf, str, sizeof(buf) - 1);
+	}
+	buf[sizeof(buf) - 1] = 0;
+	options = strchr(str, ',');
+	if (options)
+		*(options++) = 0;
+#ifdef __sparc__
+	if (!strcmp(str, "ttya"))
+		strcpy(buf, "ttyS0");
+	if (!strcmp(str, "ttyb"))
+		strcpy(buf, "ttyS1");
+#endif
+	for (s = buf; *s; s++)
+		if (isdigit(*s) || *s == ',')
+			break;
+	idx = simple_strtoul(s, NULL, 10);
+	*s = 0;
+
+	__add_preferred_console(buf, idx, options, brl_options, true);
+	console_set_on_cmdline = 1;
+	return 1;
+}
+__setup("console=", console_setup);
+
+/**
+ * add_preferred_console - add a device to the list of preferred consoles.
+ * @name: device name
+ * @idx: device index
+ * @options: options for this console
+ *
+ * The last preferred console added will be used for kernel messages
+ * and stdin/out/err for init.  Normally this is used by console_setup
+ * above to handle user-supplied console arguments; however it can also
+ * be used by arch-specific code either to override the user or more
+ * commonly to provide a default console (ie from PROM variables) when
+ * the user has not supplied one.
+ */
+int add_preferred_console(char *name, int idx, char *options)
+{
+	return __add_preferred_console(name, idx, options, NULL, false);
+}
+
+bool console_suspend_enabled = true;
+EXPORT_SYMBOL(console_suspend_enabled);
+
+static int __init console_suspend_disable(char *str)
+{
+	console_suspend_enabled = false;
+	return 1;
+}
+__setup("no_console_suspend", console_suspend_disable);
+module_param_named(console_suspend, console_suspend_enabled,
+		bool, S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(console_suspend, "suspend console during suspend"
+	" and hibernate operations");
+
+/**
+ * suspend_console - suspend the console subsystem
+ *
+ * This disables printk() while we go into suspend states
+ */
+void suspend_console(void)
+{
+	if (!console_suspend_enabled)
+		return;
+	pr_info("Suspending console(s) (use no_console_suspend to debug)\n");
+	console_lock();
+	console_suspended = 1;
+	up_console_sem();
+}
+
+void resume_console(void)
+{
+	if (!console_suspend_enabled)
+		return;
+	down_console_sem();
+	console_suspended = 0;
+	console_unlock();
+}
+
+/**
+ * console_cpu_notify - print deferred console messages after CPU hotplug
+ * @cpu: unused
+ *
+ * If printk() is called from a CPU that is not online yet, the messages
+ * will be printed on the console only if there are CON_ANYTIME consoles.
+ * This function is called when a new CPU comes online (or fails to come
+ * up) or goes offline.
+ */
+static int console_cpu_notify(unsigned int cpu)
+{
+	if (!cpuhp_tasks_frozen) {
+		/* If trylock fails, someone else is doing the printing */
+		if (console_trylock())
+			console_unlock();
+	}
+	return 0;
+}
+
+/**
+ * console_lock - lock the console system for exclusive use.
+ *
+ * Acquires a lock which guarantees that the caller has
+ * exclusive access to the console system and the console_drivers list.
+ *
+ * Can sleep, returns nothing.
+ */
+void console_lock(void)
+{
+	might_sleep();
+
+	down_console_sem();
+	if (console_suspended)
+		return;
+	console_locked = 1;
+	console_may_schedule = 1;
+}
+EXPORT_SYMBOL(console_lock);
+
+/**
+ * console_trylock - try to lock the console system for exclusive use.
+ *
+ * Try to acquire a lock which guarantees that the caller has exclusive
+ * access to the console system and the console_drivers list.
+ *
+ * returns 1 on success, and 0 on failure to acquire the lock.
+ */
+int console_trylock(void)
+{
+	if (down_trylock_console_sem())
+		return 0;
+	if (console_suspended) {
+		up_console_sem();
+		return 0;
+	}
+	console_locked = 1;
+	console_may_schedule = 0;
+	return 1;
+}
+EXPORT_SYMBOL(console_trylock);
+
+int is_console_locked(void)
+{
+	return console_locked;
+}
+EXPORT_SYMBOL(is_console_locked);
+
+/*
+ * Check if we have any console that is capable of printing while cpu is
+ * booting or shutting down. Requires console_sem.
+ */
+static int have_callable_console(void)
+{
+	struct console *con;
+
+	for_each_console(con)
+		if ((con->flags & CON_ENABLED) &&
+				(con->flags & CON_ANYTIME))
+			return 1;
+
+	return 0;
+}
+
+/*
+ * Can we actually use the console at this time on this cpu?
+ *
+ * Console drivers may assume that per-cpu resources have been allocated. So
+ * unless they're explicitly marked as being able to cope (CON_ANYTIME) don't
+ * call them until this CPU is officially up.
+ */
+static inline int can_use_console(void)
+{
+	return cpu_online(raw_smp_processor_id()) || have_callable_console();
+}
+
+/**
+ * console_unlock - unlock the console system
+ *
+ * Releases the console_lock which the caller holds on the console system
+ * and the console driver list.
+ *
+ * While the console_lock was held, console output may have been buffered
+ * by printk().  If this is the case, console_unlock(); emits
+ * the output prior to releasing the lock.
+ *
+ * If there is output waiting, we wake /dev/kmsg and syslog() users.
+ *
+ * console_unlock(); may be called from any context.
+ */
+void console_unlock(void)
+{
+	static char ext_text[CONSOLE_EXT_LOG_MAX];
+	static char text[LOG_LINE_MAX + PREFIX_MAX];
+	unsigned long flags;
+	bool do_cond_resched, retry;
+	struct printk_info info;
+	struct printk_record r;
+
+	if (console_suspended) {
+		up_console_sem();
+		return;
+	}
+
+	prb_rec_init_rd(&r, &info, text, sizeof(text));
+
+	/*
+	 * Console drivers are called with interrupts disabled, so
+	 * @console_may_schedule should be cleared before; however, we may
+	 * end up dumping a lot of lines, for example, if called from
+	 * console registration path, and should invoke cond_resched()
+	 * between lines if allowable.  Not doing so can cause a very long
+	 * scheduling stall on a slow console leading to RCU stall and
+	 * softlockup warnings which exacerbate the issue with more
+	 * messages practically incapacitating the system.
+	 *
+	 * console_trylock() is not able to detect the preemptive
+	 * context reliably. Therefore the value must be stored before
+	 * and cleared after the "again" goto label.
+	 */
+	do_cond_resched = console_may_schedule;
+again:
+	console_may_schedule = 0;
+
+	/*
+	 * We released the console_sem lock, so we need to recheck if
+	 * cpu is online and (if not) is there at least one CON_ANYTIME
+	 * console.
+	 */
+	if (!can_use_console()) {
+		console_locked = 0;
+		up_console_sem();
+		return;
+	}
+
+	for (;;) {
+		size_t ext_len = 0;
+		size_t len;
+
+		printk_safe_enter_irqsave(flags);
+		raw_spin_lock(&logbuf_lock);
+skip:
+		if (!prb_read_valid(prb, console_seq, &r))
+			break;
+
+		if (console_seq != r.info->seq) {
+			console_dropped += r.info->seq - console_seq;
+			console_seq = r.info->seq;
+		}
+
+		if (suppress_message_printing(r.info->level)) {
+			/*
+			 * Skip record we have buffered and already printed
+			 * directly to the console when we received it, and
+			 * record that has level above the console loglevel.
+			 */
+			console_seq++;
+			goto skip;
+		}
+
+		/* Output to all consoles once old messages replayed. */
+		if (unlikely(exclusive_console &&
+			     console_seq >= exclusive_console_stop_seq)) {
+			exclusive_console = NULL;
+		}
+
+		/*
+		 * Handle extended console text first because later
+		 * record_print_text() will modify the record buffer in-place.
+		 */
+		if (nr_ext_console_drivers) {
+			ext_len = info_print_ext_header(ext_text,
+						sizeof(ext_text),
+						r.info);
+			ext_len += msg_print_ext_body(ext_text + ext_len,
+						sizeof(ext_text) - ext_len,
+						&r.text_buf[0],
+						r.info->text_len,
+						&r.info->dev_info);
+		}
+		len = record_print_text(&r,
+				console_msg_format & MSG_FORMAT_SYSLOG,
+				printk_time);
+		console_seq++;
+		raw_spin_unlock(&logbuf_lock);
+
+		/*
+		 * While actively printing out messages, if another printk()
+		 * were to occur on another CPU, it may wait for this one to
+		 * finish. This task can not be preempted if there is a
+		 * waiter waiting to take over.
+		 */
+		console_lock_spinning_enable();
+
+		stop_critical_timings();	/* don't trace print latency */
+		call_console_drivers(ext_text, ext_len, text, len);
+		start_critical_timings();
+
+		if (console_lock_spinning_disable_and_check()) {
+			printk_safe_exit_irqrestore(flags);
+			return;
+		}
+
+		printk_safe_exit_irqrestore(flags);
+
+		if (do_cond_resched)
+			cond_resched();
+	}
+
+	console_locked = 0;
+
+	raw_spin_unlock(&logbuf_lock);
+
+	up_console_sem();
+
+	/*
+	 * Someone could have filled up the buffer again, so re-check if there's
+	 * something to flush. In case we cannot trylock the console_sem again,
+	 * there's a new owner and the console_unlock() from them will do the
+	 * flush, no worries.
+	 */
+	raw_spin_lock(&logbuf_lock);
+	retry = prb_read_valid(prb, console_seq, NULL);
+	raw_spin_unlock(&logbuf_lock);
+	printk_safe_exit_irqrestore(flags);
+
+	if (retry && console_trylock())
+		goto again;
+}
+EXPORT_SYMBOL(console_unlock);
+
+/**
+ * console_conditional_schedule - yield the CPU if required
+ *
+ * If the console code is currently allowed to sleep, and
+ * if this CPU should yield the CPU to another task, do
+ * so here.
+ *
+ * Must be called within console_lock();.
+ */
+void __sched console_conditional_schedule(void)
+{
+	if (console_may_schedule)
+		cond_resched();
+}
+EXPORT_SYMBOL(console_conditional_schedule);
+
+void console_unblank(void)
+{
+	struct console *c;
+
+	/*
+	 * console_unblank can no longer be called in interrupt context unless
+	 * oops_in_progress is set to 1..
+	 */
+	if (oops_in_progress) {
+		if (down_trylock_console_sem() != 0)
+			return;
+	} else
+		console_lock();
+
+	console_locked = 1;
+	console_may_schedule = 0;
+	for_each_console(c)
+		if ((c->flags & CON_ENABLED) && c->unblank)
+			c->unblank();
+	console_unlock();
+}
+
+/**
+ * console_flush_on_panic - flush console content on panic
+ * @mode: flush all messages in buffer or just the pending ones
+ *
+ * Immediately output all pending messages no matter what.
+ */
+void console_flush_on_panic(enum con_flush_mode mode)
+{
+	/*
+	 * If someone else is holding the console lock, trylock will fail
+	 * and may_schedule may be set.  Ignore and proceed to unlock so
+	 * that messages are flushed out.  As this can be called from any
+	 * context and we don't want to get preempted while flushing,
+	 * ensure may_schedule is cleared.
+	 */
+	console_trylock();
+	console_may_schedule = 0;
+
+	if (mode == CONSOLE_REPLAY_ALL) {
+		unsigned long flags;
+
+		logbuf_lock_irqsave(flags);
+		console_seq = prb_first_valid_seq(prb);
+		logbuf_unlock_irqrestore(flags);
+	}
+	console_unlock();
+}
+
+/*
+ * Return the console tty driver structure and its associated index
+ */
+struct tty_driver *console_device(int *index)
+{
+	struct console *c;
+	struct tty_driver *driver = NULL;
+
+	console_lock();
+	for_each_console(c) {
+		if (!c->device)
+			continue;
+		driver = c->device(c, index);
+		if (driver)
+			break;
+	}
+	console_unlock();
+	return driver;
+}
+
+/*
+ * Prevent further output on the passed console device so that (for example)
+ * serial drivers can disable console output before suspending a port, and can
+ * re-enable output afterwards.
+ */
+void console_stop(struct console *console)
+{
+	console_lock();
+	console->flags &= ~CON_ENABLED;
+	console_unlock();
+}
+EXPORT_SYMBOL(console_stop);
+
+void console_start(struct console *console)
+{
+	console_lock();
+	console->flags |= CON_ENABLED;
+	console_unlock();
+}
+EXPORT_SYMBOL(console_start);
+
+static int __read_mostly keep_bootcon;
+
+static int __init keep_bootcon_setup(char *str)
+{
+	keep_bootcon = 1;
+	pr_info("debug: skip boot console de-registration.\n");
+
+	return 0;
+}
+
+early_param("keep_bootcon", keep_bootcon_setup);
+
+/*
+ * This is called by register_console() to try to match
+ * the newly registered console with any of the ones selected
+ * by either the command line or add_preferred_console() and
+ * setup/enable it.
+ *
+ * Care need to be taken with consoles that are statically
+ * enabled such as netconsole
+ */
+static int try_enable_new_console(struct console *newcon, bool user_specified)
+{
+	struct console_cmdline *c;
+	int i, err;
+
+	for (i = 0, c = console_cmdline;
+	     i < MAX_CMDLINECONSOLES && c->name[0];
+	     i++, c++) {
+		if (c->user_specified != user_specified)
+			continue;
+		if (!newcon->match ||
+		    newcon->match(newcon, c->name, c->index, c->options) != 0) {
+			/* default matching */
+			BUILD_BUG_ON(sizeof(c->name) != sizeof(newcon->name));
+			if (strcmp(c->name, newcon->name) != 0)
+				continue;
+			if (newcon->index >= 0 &&
+			    newcon->index != c->index)
+				continue;
+			if (newcon->index < 0)
+				newcon->index = c->index;
+
+			if (_braille_register_console(newcon, c))
+				return 0;
+
+			if (newcon->setup &&
+			    (err = newcon->setup(newcon, c->options)) != 0)
+				return err;
+		}
+		newcon->flags |= CON_ENABLED;
+		if (i == preferred_console) {
+			newcon->flags |= CON_CONSDEV;
+			has_preferred_console = true;
+		}
+		return 0;
+	}
+
+	/*
+	 * Some consoles, such as pstore and netconsole, can be enabled even
+	 * without matching. Accept the pre-enabled consoles only when match()
+	 * and setup() had a chance to be called.
+	 */
+	if (newcon->flags & CON_ENABLED && c->user_specified ==	user_specified)
+		return 0;
+
+	return -ENOENT;
+}
+
+/*
+ * The console driver calls this routine during kernel initialization
+ * to register the console printing procedure with printk() and to
+ * print any messages that were printed by the kernel before the
+ * console driver was initialized.
+ *
+ * This can happen pretty early during the boot process (because of
+ * early_printk) - sometimes before setup_arch() completes - be careful
+ * of what kernel features are used - they may not be initialised yet.
+ *
+ * There are two types of consoles - bootconsoles (early_printk) and
+ * "real" consoles (everything which is not a bootconsole) which are
+ * handled differently.
+ *  - Any number of bootconsoles can be registered at any time.
+ *  - As soon as a "real" console is registered, all bootconsoles
+ *    will be unregistered automatically.
+ *  - Once a "real" console is registered, any attempt to register a
+ *    bootconsoles will be rejected
+ */
+void register_console(struct console *newcon)
+{
+	unsigned long flags;
+	struct console *bcon = NULL;
+	int err;
+
+	for_each_console(bcon) {
+		if (WARN(bcon == newcon, "console '%s%d' already registered\n",
+					 bcon->name, bcon->index))
+			return;
+	}
+
+	/*
+	 * before we register a new CON_BOOT console, make sure we don't
+	 * already have a valid console
+	 */
+	if (newcon->flags & CON_BOOT) {
+		for_each_console(bcon) {
+			if (!(bcon->flags & CON_BOOT)) {
+				pr_info("Too late to register bootconsole %s%d\n",
+					newcon->name, newcon->index);
+				return;
+			}
+		}
+	}
+
+	if (console_drivers && console_drivers->flags & CON_BOOT)
+		bcon = console_drivers;
+
+	if (!has_preferred_console || bcon || !console_drivers)
+		has_preferred_console = preferred_console >= 0;
+
+	/*
+	 *	See if we want to use this console driver. If we
+	 *	didn't select a console we take the first one
+	 *	that registers here.
+	 */
+	if (!has_preferred_console) {
+		if (newcon->index < 0)
+			newcon->index = 0;
+		if (newcon->setup == NULL ||
+		    newcon->setup(newcon, NULL) == 0) {
+			newcon->flags |= CON_ENABLED;
+			if (newcon->device) {
+				newcon->flags |= CON_CONSDEV;
+				has_preferred_console = true;
+			}
+		}
+	}
+
+	/* See if this console matches one we selected on the command line */
+	err = try_enable_new_console(newcon, true);
+
+	/* If not, try to match against the platform default(s) */
+	if (err == -ENOENT)
+		err = try_enable_new_console(newcon, false);
+
+	/* printk() messages are not printed to the Braille console. */
+	if (err || newcon->flags & CON_BRL)
+		return;
+
+	/*
+	 * If we have a bootconsole, and are switching to a real console,
+	 * don't print everything out again, since when the boot console, and
+	 * the real console are the same physical device, it's annoying to
+	 * see the beginning boot messages twice
+	 */
+	if (bcon && ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV))
+		newcon->flags &= ~CON_PRINTBUFFER;
+
+	/*
+	 *	Put this console in the list - keep the
+	 *	preferred driver at the head of the list.
+	 */
+	console_lock();
+	if ((newcon->flags & CON_CONSDEV) || console_drivers == NULL) {
+		newcon->next = console_drivers;
+		console_drivers = newcon;
+		if (newcon->next)
+			newcon->next->flags &= ~CON_CONSDEV;
+		/* Ensure this flag is always set for the head of the list */
+		newcon->flags |= CON_CONSDEV;
+	} else {
+		newcon->next = console_drivers->next;
+		console_drivers->next = newcon;
+	}
+
+	if (newcon->flags & CON_EXTENDED)
+		nr_ext_console_drivers++;
+
+	if (newcon->flags & CON_PRINTBUFFER) {
+		/*
+		 * console_unlock(); will print out the buffered messages
+		 * for us.
+		 */
+		logbuf_lock_irqsave(flags);
+		/*
+		 * We're about to replay the log buffer.  Only do this to the
+		 * just-registered console to avoid excessive message spam to
+		 * the already-registered consoles.
+		 *
+		 * Set exclusive_console with disabled interrupts to reduce
+		 * race window with eventual console_flush_on_panic() that
+		 * ignores console_lock.
+		 */
+		exclusive_console = newcon;
+		exclusive_console_stop_seq = console_seq;
+		console_seq = syslog_seq;
+		logbuf_unlock_irqrestore(flags);
+	}
+	console_unlock();
+	console_sysfs_notify();
+
+	/*
+	 * By unregistering the bootconsoles after we enable the real console
+	 * we get the "console xxx enabled" message on all the consoles -
+	 * boot consoles, real consoles, etc - this is to ensure that end
+	 * users know there might be something in the kernel's log buffer that
+	 * went to the bootconsole (that they do not see on the real console)
+	 */
+	pr_info("%sconsole [%s%d] enabled\n",
+		(newcon->flags & CON_BOOT) ? "boot" : "" ,
+		newcon->name, newcon->index);
+	if (bcon &&
+	    ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV) &&
+	    !keep_bootcon) {
+		/* We need to iterate through all boot consoles, to make
+		 * sure we print everything out, before we unregister them.
+		 */
+		for_each_console(bcon)
+			if (bcon->flags & CON_BOOT)
+				unregister_console(bcon);
+	}
+}
+EXPORT_SYMBOL(register_console);
+
+int unregister_console(struct console *console)
+{
+	struct console *con;
+	int res;
+
+	pr_info("%sconsole [%s%d] disabled\n",
+		(console->flags & CON_BOOT) ? "boot" : "" ,
+		console->name, console->index);
+
+	res = _braille_unregister_console(console);
+	if (res < 0)
+		return res;
+	if (res > 0)
+		return 0;
+
+	res = -ENODEV;
+	console_lock();
+	if (console_drivers == console) {
+		console_drivers=console->next;
+		res = 0;
+	} else {
+		for_each_console(con) {
+			if (con->next == console) {
+				con->next = console->next;
+				res = 0;
+				break;
+			}
+		}
+	}
+
+	if (res)
+		goto out_disable_unlock;
+
+	if (console->flags & CON_EXTENDED)
+		nr_ext_console_drivers--;
+
+	/*
+	 * If this isn't the last console and it has CON_CONSDEV set, we
+	 * need to set it on the next preferred console.
+	 */
+	if (console_drivers != NULL && console->flags & CON_CONSDEV)
+		console_drivers->flags |= CON_CONSDEV;
+
+	console->flags &= ~CON_ENABLED;
+	console_unlock();
+	console_sysfs_notify();
+
+	if (console->exit)
+		res = console->exit(console);
+
+	return res;
+
+out_disable_unlock:
+	console->flags &= ~CON_ENABLED;
+	console_unlock();
+
+	return res;
+}
+EXPORT_SYMBOL(unregister_console);
+
+/*
+ * Initialize the console device. This is called *early*, so
+ * we can't necessarily depend on lots of kernel help here.
+ * Just do some early initializations, and do the complex setup
+ * later.
+ */
+void __init console_init(void)
+{
+	int ret;
+	initcall_t call;
+	initcall_entry_t *ce;
+
+	/* Setup the default TTY line discipline. */
+	n_tty_init();
+
+	/*
+	 * set up the console device so that later boot sequences can
+	 * inform about problems etc..
+	 */
+	ce = __con_initcall_start;
+	trace_initcall_level("console");
+	while (ce < __con_initcall_end) {
+		call = initcall_from_entry(ce);
+		trace_initcall_start(call);
+		ret = call();
+		trace_initcall_finish(call, ret);
+		ce++;
+	}
+}
+
+/*
+ * Some boot consoles access data that is in the init section and which will
+ * be discarded after the initcalls have been run. To make sure that no code
+ * will access this data, unregister the boot consoles in a late initcall.
+ *
+ * If for some reason, such as deferred probe or the driver being a loadable
+ * module, the real console hasn't registered yet at this point, there will
+ * be a brief interval in which no messages are logged to the console, which
+ * makes it difficult to diagnose problems that occur during this time.
+ *
+ * To mitigate this problem somewhat, only unregister consoles whose memory
+ * intersects with the init section. Note that all other boot consoles will
+ * get unregistred when the real preferred console is registered.
+ */
+static int __init printk_late_init(void)
+{
+	struct console *con;
+	int ret;
+
+	for_each_console(con) {
+		if (!(con->flags & CON_BOOT))
+			continue;
+
+		/* Check addresses that might be used for enabled consoles. */
+		if (init_section_intersects(con, sizeof(*con)) ||
+		    init_section_contains(con->write, 0) ||
+		    init_section_contains(con->read, 0) ||
+		    init_section_contains(con->device, 0) ||
+		    init_section_contains(con->unblank, 0) ||
+		    init_section_contains(con->data, 0)) {
+			/*
+			 * Please, consider moving the reported consoles out
+			 * of the init section.
+			 */
+			pr_warn("bootconsole [%s%d] uses init memory and must be disabled even before the real one is ready\n",
+				con->name, con->index);
+			unregister_console(con);
+		}
+	}
+	ret = cpuhp_setup_state_nocalls(CPUHP_PRINTK_DEAD, "printk:dead", NULL,
+					console_cpu_notify);
+	WARN_ON(ret < 0);
+	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "printk:online",
+					console_cpu_notify, NULL);
+	WARN_ON(ret < 0);
+	return 0;
+}
+late_initcall(printk_late_init);
+
+#if defined CONFIG_PRINTK
+/*
+ * Delayed printk version, for scheduler-internal messages:
+ */
+#define PRINTK_PENDING_WAKEUP	0x01
+#define PRINTK_PENDING_OUTPUT	0x02
+
+static DEFINE_PER_CPU(int, printk_pending);
+
+static void wake_up_klogd_work_func(struct irq_work *irq_work)
+{
+	int pending = __this_cpu_xchg(printk_pending, 0);
+
+	if (pending & PRINTK_PENDING_OUTPUT) {
+		/* If trylock fails, someone else is doing the printing */
+		if (console_trylock())
+			console_unlock();
+	}
+
+	if (pending & PRINTK_PENDING_WAKEUP)
+		wake_up_interruptible(&log_wait);
+}
+
+static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = {
+	.func = wake_up_klogd_work_func,
+	.flags = ATOMIC_INIT(IRQ_WORK_LAZY),
+};
+
+void wake_up_klogd(void)
+{
+	if (!printk_percpu_data_ready())
+		return;
+
+	preempt_disable();
+	if (waitqueue_active(&log_wait)) {
+		this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
+		irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
+	}
+	preempt_enable();
+}
+
+void defer_console_output(void)
+{
+	if (!printk_percpu_data_ready())
+		return;
+
+	preempt_disable();
+	__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
+	irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
+	preempt_enable();
+}
+
+int vprintk_deferred(const char *fmt, va_list args)
+{
+	int r;
+
+	r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, fmt, args);
+	defer_console_output();
+
+	return r;
+}
+
+int printk_deferred(const char *fmt, ...)
+{
+	va_list args;
+	int r;
+
+	va_start(args, fmt);
+	r = vprintk_deferred(fmt, args);
+	va_end(args);
+
+	return r;
+}
+
+/*
+ * printk rate limiting, lifted from the networking subsystem.
+ *
+ * This enforces a rate limit: not more than 10 kernel messages
+ * every 5s to make a denial-of-service attack impossible.
+ */
+DEFINE_RATELIMIT_STATE(printk_ratelimit_state, 5 * HZ, 10);
+
+int __printk_ratelimit(const char *func)
+{
+	return ___ratelimit(&printk_ratelimit_state, func);
+}
+EXPORT_SYMBOL(__printk_ratelimit);
+
+/**
+ * printk_timed_ratelimit - caller-controlled printk ratelimiting
+ * @caller_jiffies: pointer to caller's state
+ * @interval_msecs: minimum interval between prints
+ *
+ * printk_timed_ratelimit() returns true if more than @interval_msecs
+ * milliseconds have elapsed since the last time printk_timed_ratelimit()
+ * returned true.
+ */
+bool printk_timed_ratelimit(unsigned long *caller_jiffies,
+			unsigned int interval_msecs)
+{
+	unsigned long elapsed = jiffies - *caller_jiffies;
+
+	if (*caller_jiffies && elapsed <= msecs_to_jiffies(interval_msecs))
+		return false;
+
+	*caller_jiffies = jiffies;
+	return true;
+}
+EXPORT_SYMBOL(printk_timed_ratelimit);
+
+static DEFINE_SPINLOCK(dump_list_lock);
+static LIST_HEAD(dump_list);
+
+/**
+ * kmsg_dump_register - register a kernel log dumper.
+ * @dumper: pointer to the kmsg_dumper structure
+ *
+ * Adds a kernel log dumper to the system. The dump callback in the
+ * structure will be called when the kernel oopses or panics and must be
+ * set. Returns zero on success and %-EINVAL or %-EBUSY otherwise.
+ */
+int kmsg_dump_register(struct kmsg_dumper *dumper)
+{
+	unsigned long flags;
+	int err = -EBUSY;
+
+	/* The dump callback needs to be set */
+	if (!dumper->dump)
+		return -EINVAL;
+
+	spin_lock_irqsave(&dump_list_lock, flags);
+	/* Don't allow registering multiple times */
+	if (!dumper->registered) {
+		dumper->registered = 1;
+		list_add_tail_rcu(&dumper->list, &dump_list);
+		err = 0;
+	}
+	spin_unlock_irqrestore(&dump_list_lock, flags);
+
+	return err;
+}
+EXPORT_SYMBOL_GPL(kmsg_dump_register);
+
+/**
+ * kmsg_dump_unregister - unregister a kmsg dumper.
+ * @dumper: pointer to the kmsg_dumper structure
+ *
+ * Removes a dump device from the system. Returns zero on success and
+ * %-EINVAL otherwise.
+ */
+int kmsg_dump_unregister(struct kmsg_dumper *dumper)
+{
+	unsigned long flags;
+	int err = -EINVAL;
+
+	spin_lock_irqsave(&dump_list_lock, flags);
+	if (dumper->registered) {
+		dumper->registered = 0;
+		list_del_rcu(&dumper->list);
+		err = 0;
+	}
+	spin_unlock_irqrestore(&dump_list_lock, flags);
+	synchronize_rcu();
+
+	return err;
+}
+EXPORT_SYMBOL_GPL(kmsg_dump_unregister);
+
+static bool always_kmsg_dump;
+module_param_named(always_kmsg_dump, always_kmsg_dump, bool, S_IRUGO | S_IWUSR);
+
+const char *kmsg_dump_reason_str(enum kmsg_dump_reason reason)
+{
+	switch (reason) {
+	case KMSG_DUMP_PANIC:
+		return "Panic";
+	case KMSG_DUMP_OOPS:
+		return "Oops";
+	case KMSG_DUMP_EMERG:
+		return "Emergency";
+	case KMSG_DUMP_SHUTDOWN:
+		return "Shutdown";
+	default:
+		return "Unknown";
+	}
+}
+EXPORT_SYMBOL_GPL(kmsg_dump_reason_str);
+
+/**
+ * kmsg_dump - dump kernel log to kernel message dumpers.
+ * @reason: the reason (oops, panic etc) for dumping
+ *
+ * Call each of the registered dumper's dump() callback, which can
+ * retrieve the kmsg records with kmsg_dump_get_line() or
+ * kmsg_dump_get_buffer().
+ */
+void kmsg_dump(enum kmsg_dump_reason reason)
+{
+	struct kmsg_dumper *dumper;
+	unsigned long flags;
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(dumper, &dump_list, list) {
+		enum kmsg_dump_reason max_reason = dumper->max_reason;
+
+		/*
+		 * If client has not provided a specific max_reason, default
+		 * to KMSG_DUMP_OOPS, unless always_kmsg_dump was set.
+		 */
+		if (max_reason == KMSG_DUMP_UNDEF) {
+			max_reason = always_kmsg_dump ? KMSG_DUMP_MAX :
+							KMSG_DUMP_OOPS;
+		}
+		if (reason > max_reason)
+			continue;
+
+		/* initialize iterator with data about the stored records */
+		dumper->active = true;
+
+		logbuf_lock_irqsave(flags);
+		dumper->cur_seq = clear_seq;
+		dumper->next_seq = prb_next_seq(prb);
+		logbuf_unlock_irqrestore(flags);
+
+		/* invoke dumper which will iterate over records */
+		dumper->dump(dumper, reason);
+
+		/* reset iterator */
+		dumper->active = false;
+	}
+	rcu_read_unlock();
+}
+
+/**
+ * kmsg_dump_get_line_nolock - retrieve one kmsg log line (unlocked version)
+ * @dumper: registered kmsg dumper
+ * @syslog: include the "<4>" prefixes
+ * @line: buffer to copy the line to
+ * @size: maximum size of the buffer
+ * @len: length of line placed into buffer
+ *
+ * Start at the beginning of the kmsg buffer, with the oldest kmsg
+ * record, and copy one record into the provided buffer.
+ *
+ * Consecutive calls will return the next available record moving
+ * towards the end of the buffer with the youngest messages.
+ *
+ * A return value of FALSE indicates that there are no more records to
+ * read.
+ *
+ * The function is similar to kmsg_dump_get_line(), but grabs no locks.
+ */
+bool kmsg_dump_get_line_nolock(struct kmsg_dumper *dumper, bool syslog,
+			       char *line, size_t size, size_t *len)
+{
+	struct printk_info info;
+	unsigned int line_count;
+	struct printk_record r;
+	size_t l = 0;
+	bool ret = false;
+
+	prb_rec_init_rd(&r, &info, line, size);
+
+	if (!dumper->active)
+		goto out;
+
+	/* Read text or count text lines? */
+	if (line) {
+		if (!prb_read_valid(prb, dumper->cur_seq, &r))
+			goto out;
+		l = record_print_text(&r, syslog, printk_time);
+	} else {
+		if (!prb_read_valid_info(prb, dumper->cur_seq,
+					 &info, &line_count)) {
+			goto out;
+		}
+		l = get_record_print_text_size(&info, line_count, syslog,
+					       printk_time);
+
+	}
+
+	dumper->cur_seq = r.info->seq + 1;
+	ret = true;
+out:
+	if (len)
+		*len = l;
+	return ret;
+}
+
+/**
+ * kmsg_dump_get_line - retrieve one kmsg log line
+ * @dumper: registered kmsg dumper
+ * @syslog: include the "<4>" prefixes
+ * @line: buffer to copy the line to
+ * @size: maximum size of the buffer
+ * @len: length of line placed into buffer
+ *
+ * Start at the beginning of the kmsg buffer, with the oldest kmsg
+ * record, and copy one record into the provided buffer.
+ *
+ * Consecutive calls will return the next available record moving
+ * towards the end of the buffer with the youngest messages.
+ *
+ * A return value of FALSE indicates that there are no more records to
+ * read.
+ */
+bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
+			char *line, size_t size, size_t *len)
+{
+	unsigned long flags;
+	bool ret;
+
+	logbuf_lock_irqsave(flags);
+	ret = kmsg_dump_get_line_nolock(dumper, syslog, line, size, len);
+	logbuf_unlock_irqrestore(flags);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(kmsg_dump_get_line);
+
+/**
+ * kmsg_dump_get_buffer - copy kmsg log lines
+ * @dumper: registered kmsg dumper
+ * @syslog: include the "<4>" prefixes
+ * @buf: buffer to copy the line to
+ * @size: maximum size of the buffer
+ * @len: length of line placed into buffer
+ *
+ * Start at the end of the kmsg buffer and fill the provided buffer
+ * with as many of the *youngest* kmsg records that fit into it.
+ * If the buffer is large enough, all available kmsg records will be
+ * copied with a single call.
+ *
+ * Consecutive calls will fill the buffer with the next block of
+ * available older records, not including the earlier retrieved ones.
+ *
+ * A return value of FALSE indicates that there are no more records to
+ * read.
+ */
+bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
+			  char *buf, size_t size, size_t *len)
+{
+	struct printk_info info;
+	unsigned int line_count;
+	struct printk_record r;
+	unsigned long flags;
+	u64 seq;
+	u64 next_seq;
+	size_t l = 0;
+	bool ret = false;
+	bool time = printk_time;
+
+	prb_rec_init_rd(&r, &info, buf, size);
+
+	if (!dumper->active || !buf || !size)
+		goto out;
+
+	logbuf_lock_irqsave(flags);
+	if (prb_read_valid_info(prb, dumper->cur_seq, &info, NULL)) {
+		if (info.seq != dumper->cur_seq) {
+			/* messages are gone, move to first available one */
+			dumper->cur_seq = info.seq;
+		}
+	}
+
+	/* last entry */
+	if (dumper->cur_seq >= dumper->next_seq) {
+		logbuf_unlock_irqrestore(flags);
+		goto out;
+	}
+
+	/* calculate length of entire buffer */
+	seq = dumper->cur_seq;
+	while (prb_read_valid_info(prb, seq, &info, &line_count)) {
+		if (r.info->seq >= dumper->next_seq)
+			break;
+		l += get_record_print_text_size(&info, line_count, syslog, time);
+		seq = r.info->seq + 1;
+	}
+
+	/* move first record forward until length fits into the buffer */
+	seq = dumper->cur_seq;
+	while (l >= size && prb_read_valid_info(prb, seq,
+						&info, &line_count)) {
+		if (r.info->seq >= dumper->next_seq)
+			break;
+		l -= get_record_print_text_size(&info, line_count, syslog, time);
+		seq = r.info->seq + 1;
+	}
+
+	/* last message in next interation */
+	next_seq = seq;
+
+	/* actually read text into the buffer now */
+	l = 0;
+	while (prb_read_valid(prb, seq, &r)) {
+		if (r.info->seq >= dumper->next_seq)
+			break;
+
+		l += record_print_text(&r, syslog, time);
+
+		/* adjust record to store to remaining buffer space */
+		prb_rec_init_rd(&r, &info, buf + l, size - l);
+
+		seq = r.info->seq + 1;
+	}
+
+	dumper->next_seq = next_seq;
+	ret = true;
+	logbuf_unlock_irqrestore(flags);
+out:
+	if (len)
+		*len = l;
+	return ret;
+}
+EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer);
+
+/**
+ * kmsg_dump_rewind_nolock - reset the iterator (unlocked version)
+ * @dumper: registered kmsg dumper
+ *
+ * Reset the dumper's iterator so that kmsg_dump_get_line() and
+ * kmsg_dump_get_buffer() can be called again and used multiple
+ * times within the same dumper.dump() callback.
+ *
+ * The function is similar to kmsg_dump_rewind(), but grabs no locks.
+ */
+void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper)
+{
+	dumper->cur_seq = clear_seq;
+	dumper->next_seq = prb_next_seq(prb);
+}
+
+/**
+ * kmsg_dump_rewind - reset the iterator
+ * @dumper: registered kmsg dumper
+ *
+ * Reset the dumper's iterator so that kmsg_dump_get_line() and
+ * kmsg_dump_get_buffer() can be called again and used multiple
+ * times within the same dumper.dump() callback.
+ */
+void kmsg_dump_rewind(struct kmsg_dumper *dumper)
+{
+	unsigned long flags;
+
+	logbuf_lock_irqsave(flags);
+	kmsg_dump_rewind_nolock(dumper);
+	logbuf_unlock_irqrestore(flags);
+}
+EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
+
+#endif
diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
new file mode 100644
index 000000000..3e8214e9b
--- /dev/null
+++ b/kernel/printk/printk_ringbuffer.c
@@ -0,0 +1,2084 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/kernel.h>
+#include <linux/irqflags.h>
+#include <linux/string.h>
+#include <linux/errno.h>
+#include <linux/bug.h>
+#include "printk_ringbuffer.h"
+
+/**
+ * DOC: printk_ringbuffer overview
+ *
+ * Data Structure
+ * --------------
+ * The printk_ringbuffer is made up of 3 internal ringbuffers:
+ *
+ *   desc_ring
+ *     A ring of descriptors and their meta data (such as sequence number,
+ *     timestamp, loglevel, etc.) as well as internal state information about
+ *     the record and logical positions specifying where in the other
+ *     ringbuffer the text strings are located.
+ *
+ *   text_data_ring
+ *     A ring of data blocks. A data block consists of an unsigned long
+ *     integer (ID) that maps to a desc_ring index followed by the text
+ *     string of the record.
+ *
+ * The internal state information of a descriptor is the key element to allow
+ * readers and writers to locklessly synchronize access to the data.
+ *
+ * Implementation
+ * --------------
+ *
+ * Descriptor Ring
+ * ~~~~~~~~~~~~~~~
+ * The descriptor ring is an array of descriptors. A descriptor contains
+ * essential meta data to track the data of a printk record using
+ * blk_lpos structs pointing to associated text data blocks (see
+ * "Data Rings" below). Each descriptor is assigned an ID that maps
+ * directly to index values of the descriptor array and has a state. The ID
+ * and the state are bitwise combined into a single descriptor field named
+ * @state_var, allowing ID and state to be synchronously and atomically
+ * updated.
+ *
+ * Descriptors have four states:
+ *
+ *   reserved
+ *     A writer is modifying the record.
+ *
+ *   committed
+ *     The record and all its data are written. A writer can reopen the
+ *     descriptor (transitioning it back to reserved), but in the committed
+ *     state the data is consistent.
+ *
+ *   finalized
+ *     The record and all its data are complete and available for reading. A
+ *     writer cannot reopen the descriptor.
+ *
+ *   reusable
+ *     The record exists, but its text and/or meta data may no longer be
+ *     available.
+ *
+ * Querying the @state_var of a record requires providing the ID of the
+ * descriptor to query. This can yield a possible fifth (pseudo) state:
+ *
+ *   miss
+ *     The descriptor being queried has an unexpected ID.
+ *
+ * The descriptor ring has a @tail_id that contains the ID of the oldest
+ * descriptor and @head_id that contains the ID of the newest descriptor.
+ *
+ * When a new descriptor should be created (and the ring is full), the tail
+ * descriptor is invalidated by first transitioning to the reusable state and
+ * then invalidating all tail data blocks up to and including the data blocks
+ * associated with the tail descriptor (for the text ring). Then
+ * @tail_id is advanced, followed by advancing @head_id. And finally the
+ * @state_var of the new descriptor is initialized to the new ID and reserved
+ * state.
+ *
+ * The @tail_id can only be advanced if the new @tail_id would be in the
+ * committed or reusable queried state. This makes it possible that a valid
+ * sequence number of the tail is always available.
+ *
+ * Descriptor Finalization
+ * ~~~~~~~~~~~~~~~~~~~~~~~
+ * When a writer calls the commit function prb_commit(), record data is
+ * fully stored and is consistent within the ringbuffer. However, a writer can
+ * reopen that record, claiming exclusive access (as with prb_reserve()), and
+ * modify that record. When finished, the writer must again commit the record.
+ *
+ * In order for a record to be made available to readers (and also become
+ * recyclable for writers), it must be finalized. A finalized record cannot be
+ * reopened and can never become "unfinalized". Record finalization can occur
+ * in three different scenarios:
+ *
+ *   1) A writer can simultaneously commit and finalize its record by calling
+ *      prb_final_commit() instead of prb_commit().
+ *
+ *   2) When a new record is reserved and the previous record has been
+ *      committed via prb_commit(), that previous record is automatically
+ *      finalized.
+ *
+ *   3) When a record is committed via prb_commit() and a newer record
+ *      already exists, the record being committed is automatically finalized.
+ *
+ * Data Ring
+ * ~~~~~~~~~
+ * The text data ring is a byte array composed of data blocks. Data blocks are
+ * referenced by blk_lpos structs that point to the logical position of the
+ * beginning of a data block and the beginning of the next adjacent data
+ * block. Logical positions are mapped directly to index values of the byte
+ * array ringbuffer.
+ *
+ * Each data block consists of an ID followed by the writer data. The ID is
+ * the identifier of a descriptor that is associated with the data block. A
+ * given data block is considered valid if all of the following conditions
+ * are met:
+ *
+ *   1) The descriptor associated with the data block is in the committed
+ *      or finalized queried state.
+ *
+ *   2) The blk_lpos struct within the descriptor associated with the data
+ *      block references back to the same data block.
+ *
+ *   3) The data block is within the head/tail logical position range.
+ *
+ * If the writer data of a data block would extend beyond the end of the
+ * byte array, only the ID of the data block is stored at the logical
+ * position and the full data block (ID and writer data) is stored at the
+ * beginning of the byte array. The referencing blk_lpos will point to the
+ * ID before the wrap and the next data block will be at the logical
+ * position adjacent the full data block after the wrap.
+ *
+ * Data rings have a @tail_lpos that points to the beginning of the oldest
+ * data block and a @head_lpos that points to the logical position of the
+ * next (not yet existing) data block.
+ *
+ * When a new data block should be created (and the ring is full), tail data
+ * blocks will first be invalidated by putting their associated descriptors
+ * into the reusable state and then pushing the @tail_lpos forward beyond
+ * them. Then the @head_lpos is pushed forward and is associated with a new
+ * descriptor. If a data block is not valid, the @tail_lpos cannot be
+ * advanced beyond it.
+ *
+ * Info Array
+ * ~~~~~~~~~~
+ * The general meta data of printk records are stored in printk_info structs,
+ * stored in an array with the same number of elements as the descriptor ring.
+ * Each info corresponds to the descriptor of the same index in the
+ * descriptor ring. Info validity is confirmed by evaluating the corresponding
+ * descriptor before and after loading the info.
+ *
+ * Usage
+ * -----
+ * Here are some simple examples demonstrating writers and readers. For the
+ * examples a global ringbuffer (test_rb) is available (which is not the
+ * actual ringbuffer used by printk)::
+ *
+ *	DEFINE_PRINTKRB(test_rb, 15, 5);
+ *
+ * This ringbuffer allows up to 32768 records (2 ^ 15) and has a size of
+ * 1 MiB (2 ^ (15 + 5)) for text data.
+ *
+ * Sample writer code::
+ *
+ *	const char *textstr = "message text";
+ *	struct prb_reserved_entry e;
+ *	struct printk_record r;
+ *
+ *	// specify how much to allocate
+ *	prb_rec_init_wr(&r, strlen(textstr) + 1);
+ *
+ *	if (prb_reserve(&e, &test_rb, &r)) {
+ *		snprintf(r.text_buf, r.text_buf_size, "%s", textstr);
+ *
+ *		r.info->text_len = strlen(textstr);
+ *		r.info->ts_nsec = local_clock();
+ *		r.info->caller_id = printk_caller_id();
+ *
+ *		// commit and finalize the record
+ *		prb_final_commit(&e);
+ *	}
+ *
+ * Note that additional writer functions are available to extend a record
+ * after it has been committed but not yet finalized. This can be done as
+ * long as no new records have been reserved and the caller is the same.
+ *
+ * Sample writer code (record extending)::
+ *
+ *		// alternate rest of previous example
+ *
+ *		r.info->text_len = strlen(textstr);
+ *		r.info->ts_nsec = local_clock();
+ *		r.info->caller_id = printk_caller_id();
+ *
+ *		// commit the record (but do not finalize yet)
+ *		prb_commit(&e);
+ *	}
+ *
+ *	...
+ *
+ *	// specify additional 5 bytes text space to extend
+ *	prb_rec_init_wr(&r, 5);
+ *
+ *	// try to extend, but only if it does not exceed 32 bytes
+ *	if (prb_reserve_in_last(&e, &test_rb, &r, printk_caller_id()), 32) {
+ *		snprintf(&r.text_buf[r.info->text_len],
+ *			 r.text_buf_size - r.info->text_len, "hello");
+ *
+ *		r.info->text_len += 5;
+ *
+ *		// commit and finalize the record
+ *		prb_final_commit(&e);
+ *	}
+ *
+ * Sample reader code::
+ *
+ *	struct printk_info info;
+ *	struct printk_record r;
+ *	char text_buf[32];
+ *	u64 seq;
+ *
+ *	prb_rec_init_rd(&r, &info, &text_buf[0], sizeof(text_buf));
+ *
+ *	prb_for_each_record(0, &test_rb, &seq, &r) {
+ *		if (info.seq != seq)
+ *			pr_warn("lost %llu records\n", info.seq - seq);
+ *
+ *		if (info.text_len > r.text_buf_size) {
+ *			pr_warn("record %llu text truncated\n", info.seq);
+ *			text_buf[r.text_buf_size - 1] = 0;
+ *		}
+ *
+ *		pr_info("%llu: %llu: %s\n", info.seq, info.ts_nsec,
+ *			&text_buf[0]);
+ *	}
+ *
+ * Note that additional less convenient reader functions are available to
+ * allow complex record access.
+ *
+ * ABA Issues
+ * ~~~~~~~~~~
+ * To help avoid ABA issues, descriptors are referenced by IDs (array index
+ * values combined with tagged bits counting array wraps) and data blocks are
+ * referenced by logical positions (array index values combined with tagged
+ * bits counting array wraps). However, on 32-bit systems the number of
+ * tagged bits is relatively small such that an ABA incident is (at least
+ * theoretically) possible. For example, if 4 million maximally sized (1KiB)
+ * printk messages were to occur in NMI context on a 32-bit system, the
+ * interrupted context would not be able to recognize that the 32-bit integer
+ * completely wrapped and thus represents a different data block than the one
+ * the interrupted context expects.
+ *
+ * To help combat this possibility, additional state checking is performed
+ * (such as using cmpxchg() even though set() would suffice). These extra
+ * checks are commented as such and will hopefully catch any ABA issue that
+ * a 32-bit system might experience.
+ *
+ * Memory Barriers
+ * ~~~~~~~~~~~~~~~
+ * Multiple memory barriers are used. To simplify proving correctness and
+ * generating litmus tests, lines of code related to memory barriers
+ * (loads, stores, and the associated memory barriers) are labeled::
+ *
+ *	LMM(function:letter)
+ *
+ * Comments reference the labels using only the "function:letter" part.
+ *
+ * The memory barrier pairs and their ordering are:
+ *
+ *   desc_reserve:D / desc_reserve:B
+ *     push descriptor tail (id), then push descriptor head (id)
+ *
+ *   desc_reserve:D / data_push_tail:B
+ *     push data tail (lpos), then set new descriptor reserved (state)
+ *
+ *   desc_reserve:D / desc_push_tail:C
+ *     push descriptor tail (id), then set new descriptor reserved (state)
+ *
+ *   desc_reserve:D / prb_first_seq:C
+ *     push descriptor tail (id), then set new descriptor reserved (state)
+ *
+ *   desc_reserve:F / desc_read:D
+ *     set new descriptor id and reserved (state), then allow writer changes
+ *
+ *   data_alloc:A (or data_realloc:A) / desc_read:D
+ *     set old descriptor reusable (state), then modify new data block area
+ *
+ *   data_alloc:A (or data_realloc:A) / data_push_tail:B
+ *     push data tail (lpos), then modify new data block area
+ *
+ *   _prb_commit:B / desc_read:B
+ *     store writer changes, then set new descriptor committed (state)
+ *
+ *   desc_reopen_last:A / _prb_commit:B
+ *     set descriptor reserved (state), then read descriptor data
+ *
+ *   _prb_commit:B / desc_reserve:D
+ *     set new descriptor committed (state), then check descriptor head (id)
+ *
+ *   data_push_tail:D / data_push_tail:A
+ *     set descriptor reusable (state), then push data tail (lpos)
+ *
+ *   desc_push_tail:B / desc_reserve:D
+ *     set descriptor reusable (state), then push descriptor tail (id)
+ */
+
+#define DATA_SIZE(data_ring)		_DATA_SIZE((data_ring)->size_bits)
+#define DATA_SIZE_MASK(data_ring)	(DATA_SIZE(data_ring) - 1)
+
+#define DESCS_COUNT(desc_ring)		_DESCS_COUNT((desc_ring)->count_bits)
+#define DESCS_COUNT_MASK(desc_ring)	(DESCS_COUNT(desc_ring) - 1)
+
+/* Determine the data array index from a logical position. */
+#define DATA_INDEX(data_ring, lpos)	((lpos) & DATA_SIZE_MASK(data_ring))
+
+/* Determine the desc array index from an ID or sequence number. */
+#define DESC_INDEX(desc_ring, n)	((n) & DESCS_COUNT_MASK(desc_ring))
+
+/* Determine how many times the data array has wrapped. */
+#define DATA_WRAPS(data_ring, lpos)	((lpos) >> (data_ring)->size_bits)
+
+/* Determine if a logical position refers to a data-less block. */
+#define LPOS_DATALESS(lpos)		((lpos) & 1UL)
+#define BLK_DATALESS(blk)		(LPOS_DATALESS((blk)->begin) && \
+					 LPOS_DATALESS((blk)->next))
+
+/* Get the logical position at index 0 of the current wrap. */
+#define DATA_THIS_WRAP_START_LPOS(data_ring, lpos) \
+((lpos) & ~DATA_SIZE_MASK(data_ring))
+
+/* Get the ID for the same index of the previous wrap as the given ID. */
+#define DESC_ID_PREV_WRAP(desc_ring, id) \
+DESC_ID((id) - DESCS_COUNT(desc_ring))
+
+/*
+ * A data block: mapped directly to the beginning of the data block area
+ * specified as a logical position within the data ring.
+ *
+ * @id:   the ID of the associated descriptor
+ * @data: the writer data
+ *
+ * Note that the size of a data block is only known by its associated
+ * descriptor.
+ */
+struct prb_data_block {
+	unsigned long	id;
+	char		data[];
+};
+
+/*
+ * Return the descriptor associated with @n. @n can be either a
+ * descriptor ID or a sequence number.
+ */
+static struct prb_desc *to_desc(struct prb_desc_ring *desc_ring, u64 n)
+{
+	return &desc_ring->descs[DESC_INDEX(desc_ring, n)];
+}
+
+/*
+ * Return the printk_info associated with @n. @n can be either a
+ * descriptor ID or a sequence number.
+ */
+static struct printk_info *to_info(struct prb_desc_ring *desc_ring, u64 n)
+{
+	return &desc_ring->infos[DESC_INDEX(desc_ring, n)];
+}
+
+static struct prb_data_block *to_block(struct prb_data_ring *data_ring,
+				       unsigned long begin_lpos)
+{
+	return (void *)&data_ring->data[DATA_INDEX(data_ring, begin_lpos)];
+}
+
+/*
+ * Increase the data size to account for data block meta data plus any
+ * padding so that the adjacent data block is aligned on the ID size.
+ */
+static unsigned int to_blk_size(unsigned int size)
+{
+	struct prb_data_block *db = NULL;
+
+	size += sizeof(*db);
+	size = ALIGN(size, sizeof(db->id));
+	return size;
+}
+
+/*
+ * Sanity checker for reserve size. The ringbuffer code assumes that a data
+ * block does not exceed the maximum possible size that could fit within the
+ * ringbuffer. This function provides that basic size check so that the
+ * assumption is safe.
+ */
+static bool data_check_size(struct prb_data_ring *data_ring, unsigned int size)
+{
+	struct prb_data_block *db = NULL;
+
+	if (size == 0)
+		return true;
+
+	/*
+	 * Ensure the alignment padded size could possibly fit in the data
+	 * array. The largest possible data block must still leave room for
+	 * at least the ID of the next block.
+	 */
+	size = to_blk_size(size);
+	if (size > DATA_SIZE(data_ring) - sizeof(db->id))
+		return false;
+
+	return true;
+}
+
+/* Query the state of a descriptor. */
+static enum desc_state get_desc_state(unsigned long id,
+				      unsigned long state_val)
+{
+	if (id != DESC_ID(state_val))
+		return desc_miss;
+
+	return DESC_STATE(state_val);
+}
+
+/*
+ * Get a copy of a specified descriptor and return its queried state. If the
+ * descriptor is in an inconsistent state (miss or reserved), the caller can
+ * only expect the descriptor's @state_var field to be valid.
+ *
+ * The sequence number and caller_id can be optionally retrieved. Like all
+ * non-state_var data, they are only valid if the descriptor is in a
+ * consistent state.
+ */
+static enum desc_state desc_read(struct prb_desc_ring *desc_ring,
+				 unsigned long id, struct prb_desc *desc_out,
+				 u64 *seq_out, u32 *caller_id_out)
+{
+	struct printk_info *info = to_info(desc_ring, id);
+	struct prb_desc *desc = to_desc(desc_ring, id);
+	atomic_long_t *state_var = &desc->state_var;
+	enum desc_state d_state;
+	unsigned long state_val;
+
+	/* Check the descriptor state. */
+	state_val = atomic_long_read(state_var); /* LMM(desc_read:A) */
+	d_state = get_desc_state(id, state_val);
+	if (d_state == desc_miss || d_state == desc_reserved) {
+		/*
+		 * The descriptor is in an inconsistent state. Set at least
+		 * @state_var so that the caller can see the details of
+		 * the inconsistent state.
+		 */
+		goto out;
+	}
+
+	/*
+	 * Guarantee the state is loaded before copying the descriptor
+	 * content. This avoids copying obsolete descriptor content that might
+	 * not apply to the descriptor state. This pairs with _prb_commit:B.
+	 *
+	 * Memory barrier involvement:
+	 *
+	 * If desc_read:A reads from _prb_commit:B, then desc_read:C reads
+	 * from _prb_commit:A.
+	 *
+	 * Relies on:
+	 *
+	 * WMB from _prb_commit:A to _prb_commit:B
+	 *    matching
+	 * RMB from desc_read:A to desc_read:C
+	 */
+	smp_rmb(); /* LMM(desc_read:B) */
+
+	/*
+	 * Copy the descriptor data. The data is not valid until the
+	 * state has been re-checked. A memcpy() for all of @desc
+	 * cannot be used because of the atomic_t @state_var field.
+	 */
+	memcpy(&desc_out->text_blk_lpos, &desc->text_blk_lpos,
+	       sizeof(desc_out->text_blk_lpos)); /* LMM(desc_read:C) */
+	if (seq_out)
+		*seq_out = info->seq; /* also part of desc_read:C */
+	if (caller_id_out)
+		*caller_id_out = info->caller_id; /* also part of desc_read:C */
+
+	/*
+	 * 1. Guarantee the descriptor content is loaded before re-checking
+	 *    the state. This avoids reading an obsolete descriptor state
+	 *    that may not apply to the copied content. This pairs with
+	 *    desc_reserve:F.
+	 *
+	 *    Memory barrier involvement:
+	 *
+	 *    If desc_read:C reads from desc_reserve:G, then desc_read:E
+	 *    reads from desc_reserve:F.
+	 *
+	 *    Relies on:
+	 *
+	 *    WMB from desc_reserve:F to desc_reserve:G
+	 *       matching
+	 *    RMB from desc_read:C to desc_read:E
+	 *
+	 * 2. Guarantee the record data is loaded before re-checking the
+	 *    state. This avoids reading an obsolete descriptor state that may
+	 *    not apply to the copied data. This pairs with data_alloc:A and
+	 *    data_realloc:A.
+	 *
+	 *    Memory barrier involvement:
+	 *
+	 *    If copy_data:A reads from data_alloc:B, then desc_read:E
+	 *    reads from desc_make_reusable:A.
+	 *
+	 *    Relies on:
+	 *
+	 *    MB from desc_make_reusable:A to data_alloc:B
+	 *       matching
+	 *    RMB from desc_read:C to desc_read:E
+	 *
+	 *    Note: desc_make_reusable:A and data_alloc:B can be different
+	 *          CPUs. However, the data_alloc:B CPU (which performs the
+	 *          full memory barrier) must have previously seen
+	 *          desc_make_reusable:A.
+	 */
+	smp_rmb(); /* LMM(desc_read:D) */
+
+	/*
+	 * The data has been copied. Return the current descriptor state,
+	 * which may have changed since the load above.
+	 */
+	state_val = atomic_long_read(state_var); /* LMM(desc_read:E) */
+	d_state = get_desc_state(id, state_val);
+out:
+	atomic_long_set(&desc_out->state_var, state_val);
+	return d_state;
+}
+
+/*
+ * Take a specified descriptor out of the finalized state by attempting
+ * the transition from finalized to reusable. Either this context or some
+ * other context will have been successful.
+ */
+static void desc_make_reusable(struct prb_desc_ring *desc_ring,
+			       unsigned long id)
+{
+	unsigned long val_finalized = DESC_SV(id, desc_finalized);
+	unsigned long val_reusable = DESC_SV(id, desc_reusable);
+	struct prb_desc *desc = to_desc(desc_ring, id);
+	atomic_long_t *state_var = &desc->state_var;
+
+	atomic_long_cmpxchg_relaxed(state_var, val_finalized,
+				    val_reusable); /* LMM(desc_make_reusable:A) */
+}
+
+/*
+ * Given the text data ring, put the associated descriptor of each
+ * data block from @lpos_begin until @lpos_end into the reusable state.
+ *
+ * If there is any problem making the associated descriptor reusable, either
+ * the descriptor has not yet been finalized or another writer context has
+ * already pushed the tail lpos past the problematic data block. Regardless,
+ * on error the caller can re-load the tail lpos to determine the situation.
+ */
+static bool data_make_reusable(struct printk_ringbuffer *rb,
+			       struct prb_data_ring *data_ring,
+			       unsigned long lpos_begin,
+			       unsigned long lpos_end,
+			       unsigned long *lpos_out)
+{
+	struct prb_desc_ring *desc_ring = &rb->desc_ring;
+	struct prb_data_block *blk;
+	enum desc_state d_state;
+	struct prb_desc desc;
+	struct prb_data_blk_lpos *blk_lpos = &desc.text_blk_lpos;
+	unsigned long id;
+
+	/* Loop until @lpos_begin has advanced to or beyond @lpos_end. */
+	while ((lpos_end - lpos_begin) - 1 < DATA_SIZE(data_ring)) {
+		blk = to_block(data_ring, lpos_begin);
+
+		/*
+		 * Load the block ID from the data block. This is a data race
+		 * against a writer that may have newly reserved this data
+		 * area. If the loaded value matches a valid descriptor ID,
+		 * the blk_lpos of that descriptor will be checked to make
+		 * sure it points back to this data block. If the check fails,
+		 * the data area has been recycled by another writer.
+		 */
+		id = blk->id; /* LMM(data_make_reusable:A) */
+
+		d_state = desc_read(desc_ring, id, &desc,
+				    NULL, NULL); /* LMM(data_make_reusable:B) */
+
+		switch (d_state) {
+		case desc_miss:
+		case desc_reserved:
+		case desc_committed:
+			return false;
+		case desc_finalized:
+			/*
+			 * This data block is invalid if the descriptor
+			 * does not point back to it.
+			 */
+			if (blk_lpos->begin != lpos_begin)
+				return false;
+			desc_make_reusable(desc_ring, id);
+			break;
+		case desc_reusable:
+			/*
+			 * This data block is invalid if the descriptor
+			 * does not point back to it.
+			 */
+			if (blk_lpos->begin != lpos_begin)
+				return false;
+			break;
+		}
+
+		/* Advance @lpos_begin to the next data block. */
+		lpos_begin = blk_lpos->next;
+	}
+
+	*lpos_out = lpos_begin;
+	return true;
+}
+
+/*
+ * Advance the data ring tail to at least @lpos. This function puts
+ * descriptors into the reusable state if the tail is pushed beyond
+ * their associated data block.
+ */
+static bool data_push_tail(struct printk_ringbuffer *rb,
+			   struct prb_data_ring *data_ring,
+			   unsigned long lpos)
+{
+	unsigned long tail_lpos_new;
+	unsigned long tail_lpos;
+	unsigned long next_lpos;
+
+	/* If @lpos is from a data-less block, there is nothing to do. */
+	if (LPOS_DATALESS(lpos))
+		return true;
+
+	/*
+	 * Any descriptor states that have transitioned to reusable due to the
+	 * data tail being pushed to this loaded value will be visible to this
+	 * CPU. This pairs with data_push_tail:D.
+	 *
+	 * Memory barrier involvement:
+	 *
+	 * If data_push_tail:A reads from data_push_tail:D, then this CPU can
+	 * see desc_make_reusable:A.
+	 *
+	 * Relies on:
+	 *
+	 * MB from desc_make_reusable:A to data_push_tail:D
+	 *    matches
+	 * READFROM from data_push_tail:D to data_push_tail:A
+	 *    thus
+	 * READFROM from desc_make_reusable:A to this CPU
+	 */
+	tail_lpos = atomic_long_read(&data_ring->tail_lpos); /* LMM(data_push_tail:A) */
+
+	/*
+	 * Loop until the tail lpos is at or beyond @lpos. This condition
+	 * may already be satisfied, resulting in no full memory barrier
+	 * from data_push_tail:D being performed. However, since this CPU
+	 * sees the new tail lpos, any descriptor states that transitioned to
+	 * the reusable state must already be visible.
+	 */
+	while ((lpos - tail_lpos) - 1 < DATA_SIZE(data_ring)) {
+		/*
+		 * Make all descriptors reusable that are associated with
+		 * data blocks before @lpos.
+		 */
+		if (!data_make_reusable(rb, data_ring, tail_lpos, lpos,
+					&next_lpos)) {
+			/*
+			 * 1. Guarantee the block ID loaded in
+			 *    data_make_reusable() is performed before
+			 *    reloading the tail lpos. The failed
+			 *    data_make_reusable() may be due to a newly
+			 *    recycled data area causing the tail lpos to
+			 *    have been previously pushed. This pairs with
+			 *    data_alloc:A and data_realloc:A.
+			 *
+			 *    Memory barrier involvement:
+			 *
+			 *    If data_make_reusable:A reads from data_alloc:B,
+			 *    then data_push_tail:C reads from
+			 *    data_push_tail:D.
+			 *
+			 *    Relies on:
+			 *
+			 *    MB from data_push_tail:D to data_alloc:B
+			 *       matching
+			 *    RMB from data_make_reusable:A to
+			 *    data_push_tail:C
+			 *
+			 *    Note: data_push_tail:D and data_alloc:B can be
+			 *          different CPUs. However, the data_alloc:B
+			 *          CPU (which performs the full memory
+			 *          barrier) must have previously seen
+			 *          data_push_tail:D.
+			 *
+			 * 2. Guarantee the descriptor state loaded in
+			 *    data_make_reusable() is performed before
+			 *    reloading the tail lpos. The failed
+			 *    data_make_reusable() may be due to a newly
+			 *    recycled descriptor causing the tail lpos to
+			 *    have been previously pushed. This pairs with
+			 *    desc_reserve:D.
+			 *
+			 *    Memory barrier involvement:
+			 *
+			 *    If data_make_reusable:B reads from
+			 *    desc_reserve:F, then data_push_tail:C reads
+			 *    from data_push_tail:D.
+			 *
+			 *    Relies on:
+			 *
+			 *    MB from data_push_tail:D to desc_reserve:F
+			 *       matching
+			 *    RMB from data_make_reusable:B to
+			 *    data_push_tail:C
+			 *
+			 *    Note: data_push_tail:D and desc_reserve:F can
+			 *          be different CPUs. However, the
+			 *          desc_reserve:F CPU (which performs the
+			 *          full memory barrier) must have previously
+			 *          seen data_push_tail:D.
+			 */
+			smp_rmb(); /* LMM(data_push_tail:B) */
+
+			tail_lpos_new = atomic_long_read(&data_ring->tail_lpos
+							); /* LMM(data_push_tail:C) */
+			if (tail_lpos_new == tail_lpos)
+				return false;
+
+			/* Another CPU pushed the tail. Try again. */
+			tail_lpos = tail_lpos_new;
+			continue;
+		}
+
+		/*
+		 * Guarantee any descriptor states that have transitioned to
+		 * reusable are stored before pushing the tail lpos. A full
+		 * memory barrier is needed since other CPUs may have made
+		 * the descriptor states reusable. This pairs with
+		 * data_push_tail:A.
+		 */
+		if (atomic_long_try_cmpxchg(&data_ring->tail_lpos, &tail_lpos,
+					    next_lpos)) { /* LMM(data_push_tail:D) */
+			break;
+		}
+	}
+
+	return true;
+}
+
+/*
+ * Advance the desc ring tail. This function advances the tail by one
+ * descriptor, thus invalidating the oldest descriptor. Before advancing
+ * the tail, the tail descriptor is made reusable and all data blocks up to
+ * and including the descriptor's data block are invalidated (i.e. the data
+ * ring tail is pushed past the data block of the descriptor being made
+ * reusable).
+ */
+static bool desc_push_tail(struct printk_ringbuffer *rb,
+			   unsigned long tail_id)
+{
+	struct prb_desc_ring *desc_ring = &rb->desc_ring;
+	enum desc_state d_state;
+	struct prb_desc desc;
+
+	d_state = desc_read(desc_ring, tail_id, &desc, NULL, NULL);
+
+	switch (d_state) {
+	case desc_miss:
+		/*
+		 * If the ID is exactly 1 wrap behind the expected, it is
+		 * in the process of being reserved by another writer and
+		 * must be considered reserved.
+		 */
+		if (DESC_ID(atomic_long_read(&desc.state_var)) ==
+		    DESC_ID_PREV_WRAP(desc_ring, tail_id)) {
+			return false;
+		}
+
+		/*
+		 * The ID has changed. Another writer must have pushed the
+		 * tail and recycled the descriptor already. Success is
+		 * returned because the caller is only interested in the
+		 * specified tail being pushed, which it was.
+		 */
+		return true;
+	case desc_reserved:
+	case desc_committed:
+		return false;
+	case desc_finalized:
+		desc_make_reusable(desc_ring, tail_id);
+		break;
+	case desc_reusable:
+		break;
+	}
+
+	/*
+	 * Data blocks must be invalidated before their associated
+	 * descriptor can be made available for recycling. Invalidating
+	 * them later is not possible because there is no way to trust
+	 * data blocks once their associated descriptor is gone.
+	 */
+
+	if (!data_push_tail(rb, &rb->text_data_ring, desc.text_blk_lpos.next))
+		return false;
+
+	/*
+	 * Check the next descriptor after @tail_id before pushing the tail
+	 * to it because the tail must always be in a finalized or reusable
+	 * state. The implementation of prb_first_seq() relies on this.
+	 *
+	 * A successful read implies that the next descriptor is less than or
+	 * equal to @head_id so there is no risk of pushing the tail past the
+	 * head.
+	 */
+	d_state = desc_read(desc_ring, DESC_ID(tail_id + 1), &desc,
+			    NULL, NULL); /* LMM(desc_push_tail:A) */
+
+	if (d_state == desc_finalized || d_state == desc_reusable) {
+		/*
+		 * Guarantee any descriptor states that have transitioned to
+		 * reusable are stored before pushing the tail ID. This allows
+		 * verifying the recycled descriptor state. A full memory
+		 * barrier is needed since other CPUs may have made the
+		 * descriptor states reusable. This pairs with desc_reserve:D.
+		 */
+		atomic_long_cmpxchg(&desc_ring->tail_id, tail_id,
+				    DESC_ID(tail_id + 1)); /* LMM(desc_push_tail:B) */
+	} else {
+		/*
+		 * Guarantee the last state load from desc_read() is before
+		 * reloading @tail_id in order to see a new tail ID in the
+		 * case that the descriptor has been recycled. This pairs
+		 * with desc_reserve:D.
+		 *
+		 * Memory barrier involvement:
+		 *
+		 * If desc_push_tail:A reads from desc_reserve:F, then
+		 * desc_push_tail:D reads from desc_push_tail:B.
+		 *
+		 * Relies on:
+		 *
+		 * MB from desc_push_tail:B to desc_reserve:F
+		 *    matching
+		 * RMB from desc_push_tail:A to desc_push_tail:D
+		 *
+		 * Note: desc_push_tail:B and desc_reserve:F can be different
+		 *       CPUs. However, the desc_reserve:F CPU (which performs
+		 *       the full memory barrier) must have previously seen
+		 *       desc_push_tail:B.
+		 */
+		smp_rmb(); /* LMM(desc_push_tail:C) */
+
+		/*
+		 * Re-check the tail ID. The descriptor following @tail_id is
+		 * not in an allowed tail state. But if the tail has since
+		 * been moved by another CPU, then it does not matter.
+		 */
+		if (atomic_long_read(&desc_ring->tail_id) == tail_id) /* LMM(desc_push_tail:D) */
+			return false;
+	}
+
+	return true;
+}
+
+/* Reserve a new descriptor, invalidating the oldest if necessary. */
+static bool desc_reserve(struct printk_ringbuffer *rb, unsigned long *id_out)
+{
+	struct prb_desc_ring *desc_ring = &rb->desc_ring;
+	unsigned long prev_state_val;
+	unsigned long id_prev_wrap;
+	struct prb_desc *desc;
+	unsigned long head_id;
+	unsigned long id;
+
+	head_id = atomic_long_read(&desc_ring->head_id); /* LMM(desc_reserve:A) */
+
+	do {
+		id = DESC_ID(head_id + 1);
+		id_prev_wrap = DESC_ID_PREV_WRAP(desc_ring, id);
+
+		/*
+		 * Guarantee the head ID is read before reading the tail ID.
+		 * Since the tail ID is updated before the head ID, this
+		 * guarantees that @id_prev_wrap is never ahead of the tail
+		 * ID. This pairs with desc_reserve:D.
+		 *
+		 * Memory barrier involvement:
+		 *
+		 * If desc_reserve:A reads from desc_reserve:D, then
+		 * desc_reserve:C reads from desc_push_tail:B.
+		 *
+		 * Relies on:
+		 *
+		 * MB from desc_push_tail:B to desc_reserve:D
+		 *    matching
+		 * RMB from desc_reserve:A to desc_reserve:C
+		 *
+		 * Note: desc_push_tail:B and desc_reserve:D can be different
+		 *       CPUs. However, the desc_reserve:D CPU (which performs
+		 *       the full memory barrier) must have previously seen
+		 *       desc_push_tail:B.
+		 */
+		smp_rmb(); /* LMM(desc_reserve:B) */
+
+		if (id_prev_wrap == atomic_long_read(&desc_ring->tail_id
+						    )) { /* LMM(desc_reserve:C) */
+			/*
+			 * Make space for the new descriptor by
+			 * advancing the tail.
+			 */
+			if (!desc_push_tail(rb, id_prev_wrap))
+				return false;
+		}
+
+		/*
+		 * 1. Guarantee the tail ID is read before validating the
+		 *    recycled descriptor state. A read memory barrier is
+		 *    sufficient for this. This pairs with desc_push_tail:B.
+		 *
+		 *    Memory barrier involvement:
+		 *
+		 *    If desc_reserve:C reads from desc_push_tail:B, then
+		 *    desc_reserve:E reads from desc_make_reusable:A.
+		 *
+		 *    Relies on:
+		 *
+		 *    MB from desc_make_reusable:A to desc_push_tail:B
+		 *       matching
+		 *    RMB from desc_reserve:C to desc_reserve:E
+		 *
+		 *    Note: desc_make_reusable:A and desc_push_tail:B can be
+		 *          different CPUs. However, the desc_push_tail:B CPU
+		 *          (which performs the full memory barrier) must have
+		 *          previously seen desc_make_reusable:A.
+		 *
+		 * 2. Guarantee the tail ID is stored before storing the head
+		 *    ID. This pairs with desc_reserve:B.
+		 *
+		 * 3. Guarantee any data ring tail changes are stored before
+		 *    recycling the descriptor. Data ring tail changes can
+		 *    happen via desc_push_tail()->data_push_tail(). A full
+		 *    memory barrier is needed since another CPU may have
+		 *    pushed the data ring tails. This pairs with
+		 *    data_push_tail:B.
+		 *
+		 * 4. Guarantee a new tail ID is stored before recycling the
+		 *    descriptor. A full memory barrier is needed since
+		 *    another CPU may have pushed the tail ID. This pairs
+		 *    with desc_push_tail:C and this also pairs with
+		 *    prb_first_seq:C.
+		 *
+		 * 5. Guarantee the head ID is stored before trying to
+		 *    finalize the previous descriptor. This pairs with
+		 *    _prb_commit:B.
+		 */
+	} while (!atomic_long_try_cmpxchg(&desc_ring->head_id, &head_id,
+					  id)); /* LMM(desc_reserve:D) */
+
+	desc = to_desc(desc_ring, id);
+
+	/*
+	 * If the descriptor has been recycled, verify the old state val.
+	 * See "ABA Issues" about why this verification is performed.
+	 */
+	prev_state_val = atomic_long_read(&desc->state_var); /* LMM(desc_reserve:E) */
+	if (prev_state_val &&
+	    get_desc_state(id_prev_wrap, prev_state_val) != desc_reusable) {
+		WARN_ON_ONCE(1);
+		return false;
+	}
+
+	/*
+	 * Assign the descriptor a new ID and set its state to reserved.
+	 * See "ABA Issues" about why cmpxchg() instead of set() is used.
+	 *
+	 * Guarantee the new descriptor ID and state is stored before making
+	 * any other changes. A write memory barrier is sufficient for this.
+	 * This pairs with desc_read:D.
+	 */
+	if (!atomic_long_try_cmpxchg(&desc->state_var, &prev_state_val,
+			DESC_SV(id, desc_reserved))) { /* LMM(desc_reserve:F) */
+		WARN_ON_ONCE(1);
+		return false;
+	}
+
+	/* Now data in @desc can be modified: LMM(desc_reserve:G) */
+
+	*id_out = id;
+	return true;
+}
+
+/* Determine the end of a data block. */
+static unsigned long get_next_lpos(struct prb_data_ring *data_ring,
+				   unsigned long lpos, unsigned int size)
+{
+	unsigned long begin_lpos;
+	unsigned long next_lpos;
+
+	begin_lpos = lpos;
+	next_lpos = lpos + size;
+
+	/* First check if the data block does not wrap. */
+	if (DATA_WRAPS(data_ring, begin_lpos) == DATA_WRAPS(data_ring, next_lpos))
+		return next_lpos;
+
+	/* Wrapping data blocks store their data at the beginning. */
+	return (DATA_THIS_WRAP_START_LPOS(data_ring, next_lpos) + size);
+}
+
+/*
+ * Allocate a new data block, invalidating the oldest data block(s)
+ * if necessary. This function also associates the data block with
+ * a specified descriptor.
+ */
+static char *data_alloc(struct printk_ringbuffer *rb,
+			struct prb_data_ring *data_ring, unsigned int size,
+			struct prb_data_blk_lpos *blk_lpos, unsigned long id)
+{
+	struct prb_data_block *blk;
+	unsigned long begin_lpos;
+	unsigned long next_lpos;
+
+	if (size == 0) {
+		/* Specify a data-less block. */
+		blk_lpos->begin = NO_LPOS;
+		blk_lpos->next = NO_LPOS;
+		return NULL;
+	}
+
+	size = to_blk_size(size);
+
+	begin_lpos = atomic_long_read(&data_ring->head_lpos);
+
+	do {
+		next_lpos = get_next_lpos(data_ring, begin_lpos, size);
+
+		if (!data_push_tail(rb, data_ring, next_lpos - DATA_SIZE(data_ring))) {
+			/* Failed to allocate, specify a data-less block. */
+			blk_lpos->begin = FAILED_LPOS;
+			blk_lpos->next = FAILED_LPOS;
+			return NULL;
+		}
+
+		/*
+		 * 1. Guarantee any descriptor states that have transitioned
+		 *    to reusable are stored before modifying the newly
+		 *    allocated data area. A full memory barrier is needed
+		 *    since other CPUs may have made the descriptor states
+		 *    reusable. See data_push_tail:A about why the reusable
+		 *    states are visible. This pairs with desc_read:D.
+		 *
+		 * 2. Guarantee any updated tail lpos is stored before
+		 *    modifying the newly allocated data area. Another CPU may
+		 *    be in data_make_reusable() and is reading a block ID
+		 *    from this area. data_make_reusable() can handle reading
+		 *    a garbage block ID value, but then it must be able to
+		 *    load a new tail lpos. A full memory barrier is needed
+		 *    since other CPUs may have updated the tail lpos. This
+		 *    pairs with data_push_tail:B.
+		 */
+	} while (!atomic_long_try_cmpxchg(&data_ring->head_lpos, &begin_lpos,
+					  next_lpos)); /* LMM(data_alloc:A) */
+
+	blk = to_block(data_ring, begin_lpos);
+	blk->id = id; /* LMM(data_alloc:B) */
+
+	if (DATA_WRAPS(data_ring, begin_lpos) != DATA_WRAPS(data_ring, next_lpos)) {
+		/* Wrapping data blocks store their data at the beginning. */
+		blk = to_block(data_ring, 0);
+
+		/*
+		 * Store the ID on the wrapped block for consistency.
+		 * The printk_ringbuffer does not actually use it.
+		 */
+		blk->id = id;
+	}
+
+	blk_lpos->begin = begin_lpos;
+	blk_lpos->next = next_lpos;
+
+	return &blk->data[0];
+}
+
+/*
+ * Try to resize an existing data block associated with the descriptor
+ * specified by @id. If the resized data block should become wrapped, it
+ * copies the old data to the new data block. If @size yields a data block
+ * with the same or less size, the data block is left as is.
+ *
+ * Fail if this is not the last allocated data block or if there is not
+ * enough space or it is not possible make enough space.
+ *
+ * Return a pointer to the beginning of the entire data buffer or NULL on
+ * failure.
+ */
+static char *data_realloc(struct printk_ringbuffer *rb,
+			  struct prb_data_ring *data_ring, unsigned int size,
+			  struct prb_data_blk_lpos *blk_lpos, unsigned long id)
+{
+	struct prb_data_block *blk;
+	unsigned long head_lpos;
+	unsigned long next_lpos;
+	bool wrapped;
+
+	/* Reallocation only works if @blk_lpos is the newest data block. */
+	head_lpos = atomic_long_read(&data_ring->head_lpos);
+	if (head_lpos != blk_lpos->next)
+		return NULL;
+
+	/* Keep track if @blk_lpos was a wrapping data block. */
+	wrapped = (DATA_WRAPS(data_ring, blk_lpos->begin) != DATA_WRAPS(data_ring, blk_lpos->next));
+
+	size = to_blk_size(size);
+
+	next_lpos = get_next_lpos(data_ring, blk_lpos->begin, size);
+
+	/* If the data block does not increase, there is nothing to do. */
+	if (head_lpos - next_lpos < DATA_SIZE(data_ring)) {
+		if (wrapped)
+			blk = to_block(data_ring, 0);
+		else
+			blk = to_block(data_ring, blk_lpos->begin);
+		return &blk->data[0];
+	}
+
+	if (!data_push_tail(rb, data_ring, next_lpos - DATA_SIZE(data_ring)))
+		return NULL;
+
+	/* The memory barrier involvement is the same as data_alloc:A. */
+	if (!atomic_long_try_cmpxchg(&data_ring->head_lpos, &head_lpos,
+				     next_lpos)) { /* LMM(data_realloc:A) */
+		return NULL;
+	}
+
+	blk = to_block(data_ring, blk_lpos->begin);
+
+	if (DATA_WRAPS(data_ring, blk_lpos->begin) != DATA_WRAPS(data_ring, next_lpos)) {
+		struct prb_data_block *old_blk = blk;
+
+		/* Wrapping data blocks store their data at the beginning. */
+		blk = to_block(data_ring, 0);
+
+		/*
+		 * Store the ID on the wrapped block for consistency.
+		 * The printk_ringbuffer does not actually use it.
+		 */
+		blk->id = id;
+
+		if (!wrapped) {
+			/*
+			 * Since the allocated space is now in the newly
+			 * created wrapping data block, copy the content
+			 * from the old data block.
+			 */
+			memcpy(&blk->data[0], &old_blk->data[0],
+			       (blk_lpos->next - blk_lpos->begin) - sizeof(blk->id));
+		}
+	}
+
+	blk_lpos->next = next_lpos;
+
+	return &blk->data[0];
+}
+
+/* Return the number of bytes used by a data block. */
+static unsigned int space_used(struct prb_data_ring *data_ring,
+			       struct prb_data_blk_lpos *blk_lpos)
+{
+	/* Data-less blocks take no space. */
+	if (BLK_DATALESS(blk_lpos))
+		return 0;
+
+	if (DATA_WRAPS(data_ring, blk_lpos->begin) == DATA_WRAPS(data_ring, blk_lpos->next)) {
+		/* Data block does not wrap. */
+		return (DATA_INDEX(data_ring, blk_lpos->next) -
+			DATA_INDEX(data_ring, blk_lpos->begin));
+	}
+
+	/*
+	 * For wrapping data blocks, the trailing (wasted) space is
+	 * also counted.
+	 */
+	return (DATA_INDEX(data_ring, blk_lpos->next) +
+		DATA_SIZE(data_ring) - DATA_INDEX(data_ring, blk_lpos->begin));
+}
+
+/*
+ * Given @blk_lpos, return a pointer to the writer data from the data block
+ * and calculate the size of the data part. A NULL pointer is returned if
+ * @blk_lpos specifies values that could never be legal.
+ *
+ * This function (used by readers) performs strict validation on the lpos
+ * values to possibly detect bugs in the writer code. A WARN_ON_ONCE() is
+ * triggered if an internal error is detected.
+ */
+static const char *get_data(struct prb_data_ring *data_ring,
+			    struct prb_data_blk_lpos *blk_lpos,
+			    unsigned int *data_size)
+{
+	struct prb_data_block *db;
+
+	/* Data-less data block description. */
+	if (BLK_DATALESS(blk_lpos)) {
+		if (blk_lpos->begin == NO_LPOS && blk_lpos->next == NO_LPOS) {
+			*data_size = 0;
+			return "";
+		}
+		return NULL;
+	}
+
+	/* Regular data block: @begin less than @next and in same wrap. */
+	if (DATA_WRAPS(data_ring, blk_lpos->begin) == DATA_WRAPS(data_ring, blk_lpos->next) &&
+	    blk_lpos->begin < blk_lpos->next) {
+		db = to_block(data_ring, blk_lpos->begin);
+		*data_size = blk_lpos->next - blk_lpos->begin;
+
+	/* Wrapping data block: @begin is one wrap behind @next. */
+	} else if (DATA_WRAPS(data_ring, blk_lpos->begin + DATA_SIZE(data_ring)) ==
+		   DATA_WRAPS(data_ring, blk_lpos->next)) {
+		db = to_block(data_ring, 0);
+		*data_size = DATA_INDEX(data_ring, blk_lpos->next);
+
+	/* Illegal block description. */
+	} else {
+		WARN_ON_ONCE(1);
+		return NULL;
+	}
+
+	/* A valid data block will always be aligned to the ID size. */
+	if (WARN_ON_ONCE(blk_lpos->begin != ALIGN(blk_lpos->begin, sizeof(db->id))) ||
+	    WARN_ON_ONCE(blk_lpos->next != ALIGN(blk_lpos->next, sizeof(db->id)))) {
+		return NULL;
+	}
+
+	/* A valid data block will always have at least an ID. */
+	if (WARN_ON_ONCE(*data_size < sizeof(db->id)))
+		return NULL;
+
+	/* Subtract block ID space from size to reflect data size. */
+	*data_size -= sizeof(db->id);
+
+	return &db->data[0];
+}
+
+/*
+ * Attempt to transition the newest descriptor from committed back to reserved
+ * so that the record can be modified by a writer again. This is only possible
+ * if the descriptor is not yet finalized and the provided @caller_id matches.
+ */
+static struct prb_desc *desc_reopen_last(struct prb_desc_ring *desc_ring,
+					 u32 caller_id, unsigned long *id_out)
+{
+	unsigned long prev_state_val;
+	enum desc_state d_state;
+	struct prb_desc desc;
+	struct prb_desc *d;
+	unsigned long id;
+	u32 cid;
+
+	id = atomic_long_read(&desc_ring->head_id);
+
+	/*
+	 * To reduce unnecessarily reopening, first check if the descriptor
+	 * state and caller ID are correct.
+	 */
+	d_state = desc_read(desc_ring, id, &desc, NULL, &cid);
+	if (d_state != desc_committed || cid != caller_id)
+		return NULL;
+
+	d = to_desc(desc_ring, id);
+
+	prev_state_val = DESC_SV(id, desc_committed);
+
+	/*
+	 * Guarantee the reserved state is stored before reading any
+	 * record data. A full memory barrier is needed because @state_var
+	 * modification is followed by reading. This pairs with _prb_commit:B.
+	 *
+	 * Memory barrier involvement:
+	 *
+	 * If desc_reopen_last:A reads from _prb_commit:B, then
+	 * prb_reserve_in_last:A reads from _prb_commit:A.
+	 *
+	 * Relies on:
+	 *
+	 * WMB from _prb_commit:A to _prb_commit:B
+	 *    matching
+	 * MB If desc_reopen_last:A to prb_reserve_in_last:A
+	 */
+	if (!atomic_long_try_cmpxchg(&d->state_var, &prev_state_val,
+			DESC_SV(id, desc_reserved))) { /* LMM(desc_reopen_last:A) */
+		return NULL;
+	}
+
+	*id_out = id;
+	return d;
+}
+
+/**
+ * prb_reserve_in_last() - Re-reserve and extend the space in the ringbuffer
+ *                         used by the newest record.
+ *
+ * @e:         The entry structure to setup.
+ * @rb:        The ringbuffer to re-reserve and extend data in.
+ * @r:         The record structure to allocate buffers for.
+ * @caller_id: The caller ID of the caller (reserving writer).
+ * @max_size:  Fail if the extended size would be greater than this.
+ *
+ * This is the public function available to writers to re-reserve and extend
+ * data.
+ *
+ * The writer specifies the text size to extend (not the new total size) by
+ * setting the @text_buf_size field of @r. To ensure proper initialization
+ * of @r, prb_rec_init_wr() should be used.
+ *
+ * This function will fail if @caller_id does not match the caller ID of the
+ * newest record. In that case the caller must reserve new data using
+ * prb_reserve().
+ *
+ * Context: Any context. Disables local interrupts on success.
+ * Return: true if text data could be extended, otherwise false.
+ *
+ * On success:
+ *
+ *   - @r->text_buf points to the beginning of the entire text buffer.
+ *
+ *   - @r->text_buf_size is set to the new total size of the buffer.
+ *
+ *   - @r->info is not touched so that @r->info->text_len could be used
+ *     to append the text.
+ *
+ *   - prb_record_text_space() can be used on @e to query the new
+ *     actually used space.
+ *
+ * Important: All @r->info fields will already be set with the current values
+ *            for the record. I.e. @r->info->text_len will be less than
+ *            @text_buf_size. Writers can use @r->info->text_len to know
+ *            where concatenation begins and writers should update
+ *            @r->info->text_len after concatenating.
+ */
+bool prb_reserve_in_last(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
+			 struct printk_record *r, u32 caller_id, unsigned int max_size)
+{
+	struct prb_desc_ring *desc_ring = &rb->desc_ring;
+	struct printk_info *info;
+	unsigned int data_size;
+	struct prb_desc *d;
+	unsigned long id;
+
+	local_irq_save(e->irqflags);
+
+	/* Transition the newest descriptor back to the reserved state. */
+	d = desc_reopen_last(desc_ring, caller_id, &id);
+	if (!d) {
+		local_irq_restore(e->irqflags);
+		goto fail_reopen;
+	}
+
+	/* Now the writer has exclusive access: LMM(prb_reserve_in_last:A) */
+
+	info = to_info(desc_ring, id);
+
+	/*
+	 * Set the @e fields here so that prb_commit() can be used if
+	 * anything fails from now on.
+	 */
+	e->rb = rb;
+	e->id = id;
+
+	/*
+	 * desc_reopen_last() checked the caller_id, but there was no
+	 * exclusive access at that point. The descriptor may have
+	 * changed since then.
+	 */
+	if (caller_id != info->caller_id)
+		goto fail;
+
+	if (BLK_DATALESS(&d->text_blk_lpos)) {
+		if (WARN_ON_ONCE(info->text_len != 0)) {
+			pr_warn_once("wrong text_len value (%hu, expecting 0)\n",
+				     info->text_len);
+			info->text_len = 0;
+		}
+
+		if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
+			goto fail;
+
+		if (r->text_buf_size > max_size)
+			goto fail;
+
+		r->text_buf = data_alloc(rb, &rb->text_data_ring, r->text_buf_size,
+					 &d->text_blk_lpos, id);
+	} else {
+		if (!get_data(&rb->text_data_ring, &d->text_blk_lpos, &data_size))
+			goto fail;
+
+		/*
+		 * Increase the buffer size to include the original size. If
+		 * the meta data (@text_len) is not sane, use the full data
+		 * block size.
+		 */
+		if (WARN_ON_ONCE(info->text_len > data_size)) {
+			pr_warn_once("wrong text_len value (%hu, expecting <=%u)\n",
+				     info->text_len, data_size);
+			info->text_len = data_size;
+		}
+		r->text_buf_size += info->text_len;
+
+		if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
+			goto fail;
+
+		if (r->text_buf_size > max_size)
+			goto fail;
+
+		r->text_buf = data_realloc(rb, &rb->text_data_ring, r->text_buf_size,
+					   &d->text_blk_lpos, id);
+	}
+	if (r->text_buf_size && !r->text_buf)
+		goto fail;
+
+	r->info = info;
+
+	e->text_space = space_used(&rb->text_data_ring, &d->text_blk_lpos);
+
+	return true;
+fail:
+	prb_commit(e);
+	/* prb_commit() re-enabled interrupts. */
+fail_reopen:
+	/* Make it clear to the caller that the re-reserve failed. */
+	memset(r, 0, sizeof(*r));
+	return false;
+}
+
+/*
+ * Attempt to finalize a specified descriptor. If this fails, the descriptor
+ * is either already final or it will finalize itself when the writer commits.
+ */
+static void desc_make_final(struct prb_desc_ring *desc_ring, unsigned long id)
+{
+	unsigned long prev_state_val = DESC_SV(id, desc_committed);
+	struct prb_desc *d = to_desc(desc_ring, id);
+
+	atomic_long_cmpxchg_relaxed(&d->state_var, prev_state_val,
+			DESC_SV(id, desc_finalized)); /* LMM(desc_make_final:A) */
+}
+
+/**
+ * prb_reserve() - Reserve space in the ringbuffer.
+ *
+ * @e:  The entry structure to setup.
+ * @rb: The ringbuffer to reserve data in.
+ * @r:  The record structure to allocate buffers for.
+ *
+ * This is the public function available to writers to reserve data.
+ *
+ * The writer specifies the text size to reserve by setting the
+ * @text_buf_size field of @r. To ensure proper initialization of @r,
+ * prb_rec_init_wr() should be used.
+ *
+ * Context: Any context. Disables local interrupts on success.
+ * Return: true if at least text data could be allocated, otherwise false.
+ *
+ * On success, the fields @info and @text_buf of @r will be set by this
+ * function and should be filled in by the writer before committing. Also
+ * on success, prb_record_text_space() can be used on @e to query the actual
+ * space used for the text data block.
+ *
+ * Important: @info->text_len needs to be set correctly by the writer in
+ *            order for data to be readable and/or extended. Its value
+ *            is initialized to 0.
+ */
+bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
+		 struct printk_record *r)
+{
+	struct prb_desc_ring *desc_ring = &rb->desc_ring;
+	struct printk_info *info;
+	struct prb_desc *d;
+	unsigned long id;
+	u64 seq;
+
+	if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
+		goto fail;
+
+	/*
+	 * Descriptors in the reserved state act as blockers to all further
+	 * reservations once the desc_ring has fully wrapped. Disable
+	 * interrupts during the reserve/commit window in order to minimize
+	 * the likelihood of this happening.
+	 */
+	local_irq_save(e->irqflags);
+
+	if (!desc_reserve(rb, &id)) {
+		/* Descriptor reservation failures are tracked. */
+		atomic_long_inc(&rb->fail);
+		local_irq_restore(e->irqflags);
+		goto fail;
+	}
+
+	d = to_desc(desc_ring, id);
+	info = to_info(desc_ring, id);
+
+	/*
+	 * All @info fields (except @seq) are cleared and must be filled in
+	 * by the writer. Save @seq before clearing because it is used to
+	 * determine the new sequence number.
+	 */
+	seq = info->seq;
+	memset(info, 0, sizeof(*info));
+
+	/*
+	 * Set the @e fields here so that prb_commit() can be used if
+	 * text data allocation fails.
+	 */
+	e->rb = rb;
+	e->id = id;
+
+	/*
+	 * Initialize the sequence number if it has "never been set".
+	 * Otherwise just increment it by a full wrap.
+	 *
+	 * @seq is considered "never been set" if it has a value of 0,
+	 * _except_ for @infos[0], which was specially setup by the ringbuffer
+	 * initializer and therefore is always considered as set.
+	 *
+	 * See the "Bootstrap" comment block in printk_ringbuffer.h for
+	 * details about how the initializer bootstraps the descriptors.
+	 */
+	if (seq == 0 && DESC_INDEX(desc_ring, id) != 0)
+		info->seq = DESC_INDEX(desc_ring, id);
+	else
+		info->seq = seq + DESCS_COUNT(desc_ring);
+
+	/*
+	 * New data is about to be reserved. Once that happens, previous
+	 * descriptors are no longer able to be extended. Finalize the
+	 * previous descriptor now so that it can be made available to
+	 * readers. (For seq==0 there is no previous descriptor.)
+	 */
+	if (info->seq > 0)
+		desc_make_final(desc_ring, DESC_ID(id - 1));
+
+	r->text_buf = data_alloc(rb, &rb->text_data_ring, r->text_buf_size,
+				 &d->text_blk_lpos, id);
+	/* If text data allocation fails, a data-less record is committed. */
+	if (r->text_buf_size && !r->text_buf) {
+		prb_commit(e);
+		/* prb_commit() re-enabled interrupts. */
+		goto fail;
+	}
+
+	r->info = info;
+
+	/* Record full text space used by record. */
+	e->text_space = space_used(&rb->text_data_ring, &d->text_blk_lpos);
+
+	return true;
+fail:
+	/* Make it clear to the caller that the reserve failed. */
+	memset(r, 0, sizeof(*r));
+	return false;
+}
+
+/* Commit the data (possibly finalizing it) and restore interrupts. */
+static void _prb_commit(struct prb_reserved_entry *e, unsigned long state_val)
+{
+	struct prb_desc_ring *desc_ring = &e->rb->desc_ring;
+	struct prb_desc *d = to_desc(desc_ring, e->id);
+	unsigned long prev_state_val = DESC_SV(e->id, desc_reserved);
+
+	/* Now the writer has finished all writing: LMM(_prb_commit:A) */
+
+	/*
+	 * Set the descriptor as committed. See "ABA Issues" about why
+	 * cmpxchg() instead of set() is used.
+	 *
+	 * 1  Guarantee all record data is stored before the descriptor state
+	 *    is stored as committed. A write memory barrier is sufficient
+	 *    for this. This pairs with desc_read:B and desc_reopen_last:A.
+	 *
+	 * 2. Guarantee the descriptor state is stored as committed before
+	 *    re-checking the head ID in order to possibly finalize this
+	 *    descriptor. This pairs with desc_reserve:D.
+	 *
+	 *    Memory barrier involvement:
+	 *
+	 *    If prb_commit:A reads from desc_reserve:D, then
+	 *    desc_make_final:A reads from _prb_commit:B.
+	 *
+	 *    Relies on:
+	 *
+	 *    MB _prb_commit:B to prb_commit:A
+	 *       matching
+	 *    MB desc_reserve:D to desc_make_final:A
+	 */
+	if (!atomic_long_try_cmpxchg(&d->state_var, &prev_state_val,
+			DESC_SV(e->id, state_val))) { /* LMM(_prb_commit:B) */
+		WARN_ON_ONCE(1);
+	}
+
+	/* Restore interrupts, the reserve/commit window is finished. */
+	local_irq_restore(e->irqflags);
+}
+
+/**
+ * prb_commit() - Commit (previously reserved) data to the ringbuffer.
+ *
+ * @e: The entry containing the reserved data information.
+ *
+ * This is the public function available to writers to commit data.
+ *
+ * Note that the data is not yet available to readers until it is finalized.
+ * Finalizing happens automatically when space for the next record is
+ * reserved.
+ *
+ * See prb_final_commit() for a version of this function that finalizes
+ * immediately.
+ *
+ * Context: Any context. Enables local interrupts.
+ */
+void prb_commit(struct prb_reserved_entry *e)
+{
+	struct prb_desc_ring *desc_ring = &e->rb->desc_ring;
+	unsigned long head_id;
+
+	_prb_commit(e, desc_committed);
+
+	/*
+	 * If this descriptor is no longer the head (i.e. a new record has
+	 * been allocated), extending the data for this record is no longer
+	 * allowed and therefore it must be finalized.
+	 */
+	head_id = atomic_long_read(&desc_ring->head_id); /* LMM(prb_commit:A) */
+	if (head_id != e->id)
+		desc_make_final(desc_ring, e->id);
+}
+
+/**
+ * prb_final_commit() - Commit and finalize (previously reserved) data to
+ *                      the ringbuffer.
+ *
+ * @e: The entry containing the reserved data information.
+ *
+ * This is the public function available to writers to commit+finalize data.
+ *
+ * By finalizing, the data is made immediately available to readers.
+ *
+ * This function should only be used if there are no intentions of extending
+ * this data using prb_reserve_in_last().
+ *
+ * Context: Any context. Enables local interrupts.
+ */
+void prb_final_commit(struct prb_reserved_entry *e)
+{
+	_prb_commit(e, desc_finalized);
+}
+
+/*
+ * Count the number of lines in provided text. All text has at least 1 line
+ * (even if @text_size is 0). Each '\n' processed is counted as an additional
+ * line.
+ */
+static unsigned int count_lines(const char *text, unsigned int text_size)
+{
+	unsigned int next_size = text_size;
+	unsigned int line_count = 1;
+	const char *next = text;
+
+	while (next_size) {
+		next = memchr(next, '\n', next_size);
+		if (!next)
+			break;
+		line_count++;
+		next++;
+		next_size = text_size - (next - text);
+	}
+
+	return line_count;
+}
+
+/*
+ * Given @blk_lpos, copy an expected @len of data into the provided buffer.
+ * If @line_count is provided, count the number of lines in the data.
+ *
+ * This function (used by readers) performs strict validation on the data
+ * size to possibly detect bugs in the writer code. A WARN_ON_ONCE() is
+ * triggered if an internal error is detected.
+ */
+static bool copy_data(struct prb_data_ring *data_ring,
+		      struct prb_data_blk_lpos *blk_lpos, u16 len, char *buf,
+		      unsigned int buf_size, unsigned int *line_count)
+{
+	unsigned int data_size;
+	const char *data;
+
+	/* Caller might not want any data. */
+	if ((!buf || !buf_size) && !line_count)
+		return true;
+
+	data = get_data(data_ring, blk_lpos, &data_size);
+	if (!data)
+		return false;
+
+	/*
+	 * Actual cannot be less than expected. It can be more than expected
+	 * because of the trailing alignment padding.
+	 *
+	 * Note that invalid @len values can occur because the caller loads
+	 * the value during an allowed data race.
+	 */
+	if (data_size < (unsigned int)len)
+		return false;
+
+	/* Caller interested in the line count? */
+	if (line_count)
+		*line_count = count_lines(data, len);
+
+	/* Caller interested in the data content? */
+	if (!buf || !buf_size)
+		return true;
+
+	data_size = min_t(unsigned int, buf_size, len);
+
+	memcpy(&buf[0], data, data_size); /* LMM(copy_data:A) */
+	return true;
+}
+
+/*
+ * This is an extended version of desc_read(). It gets a copy of a specified
+ * descriptor. However, it also verifies that the record is finalized and has
+ * the sequence number @seq. On success, 0 is returned.
+ *
+ * Error return values:
+ * -EINVAL: A finalized record with sequence number @seq does not exist.
+ * -ENOENT: A finalized record with sequence number @seq exists, but its data
+ *          is not available. This is a valid record, so readers should
+ *          continue with the next record.
+ */
+static int desc_read_finalized_seq(struct prb_desc_ring *desc_ring,
+				   unsigned long id, u64 seq,
+				   struct prb_desc *desc_out)
+{
+	struct prb_data_blk_lpos *blk_lpos = &desc_out->text_blk_lpos;
+	enum desc_state d_state;
+	u64 s;
+
+	d_state = desc_read(desc_ring, id, desc_out, &s, NULL);
+
+	/*
+	 * An unexpected @id (desc_miss) or @seq mismatch means the record
+	 * does not exist. A descriptor in the reserved or committed state
+	 * means the record does not yet exist for the reader.
+	 */
+	if (d_state == desc_miss ||
+	    d_state == desc_reserved ||
+	    d_state == desc_committed ||
+	    s != seq) {
+		return -EINVAL;
+	}
+
+	/*
+	 * A descriptor in the reusable state may no longer have its data
+	 * available; report it as existing but with lost data. Or the record
+	 * may actually be a record with lost data.
+	 */
+	if (d_state == desc_reusable ||
+	    (blk_lpos->begin == FAILED_LPOS && blk_lpos->next == FAILED_LPOS)) {
+		return -ENOENT;
+	}
+
+	return 0;
+}
+
+/*
+ * Copy the ringbuffer data from the record with @seq to the provided
+ * @r buffer. On success, 0 is returned.
+ *
+ * See desc_read_finalized_seq() for error return values.
+ */
+static int prb_read(struct printk_ringbuffer *rb, u64 seq,
+		    struct printk_record *r, unsigned int *line_count)
+{
+	struct prb_desc_ring *desc_ring = &rb->desc_ring;
+	struct printk_info *info = to_info(desc_ring, seq);
+	struct prb_desc *rdesc = to_desc(desc_ring, seq);
+	atomic_long_t *state_var = &rdesc->state_var;
+	struct prb_desc desc;
+	unsigned long id;
+	int err;
+
+	/* Extract the ID, used to specify the descriptor to read. */
+	id = DESC_ID(atomic_long_read(state_var));
+
+	/* Get a local copy of the correct descriptor (if available). */
+	err = desc_read_finalized_seq(desc_ring, id, seq, &desc);
+
+	/*
+	 * If @r is NULL, the caller is only interested in the availability
+	 * of the record.
+	 */
+	if (err || !r)
+		return err;
+
+	/* If requested, copy meta data. */
+	if (r->info)
+		memcpy(r->info, info, sizeof(*(r->info)));
+
+	/* Copy text data. If it fails, this is a data-less record. */
+	if (!copy_data(&rb->text_data_ring, &desc.text_blk_lpos, info->text_len,
+		       r->text_buf, r->text_buf_size, line_count)) {
+		return -ENOENT;
+	}
+
+	/* Ensure the record is still finalized and has the same @seq. */
+	return desc_read_finalized_seq(desc_ring, id, seq, &desc);
+}
+
+/* Get the sequence number of the tail descriptor. */
+static u64 prb_first_seq(struct printk_ringbuffer *rb)
+{
+	struct prb_desc_ring *desc_ring = &rb->desc_ring;
+	enum desc_state d_state;
+	struct prb_desc desc;
+	unsigned long id;
+	u64 seq;
+
+	for (;;) {
+		id = atomic_long_read(&rb->desc_ring.tail_id); /* LMM(prb_first_seq:A) */
+
+		d_state = desc_read(desc_ring, id, &desc, &seq, NULL); /* LMM(prb_first_seq:B) */
+
+		/*
+		 * This loop will not be infinite because the tail is
+		 * _always_ in the finalized or reusable state.
+		 */
+		if (d_state == desc_finalized || d_state == desc_reusable)
+			break;
+
+		/*
+		 * Guarantee the last state load from desc_read() is before
+		 * reloading @tail_id in order to see a new tail in the case
+		 * that the descriptor has been recycled. This pairs with
+		 * desc_reserve:D.
+		 *
+		 * Memory barrier involvement:
+		 *
+		 * If prb_first_seq:B reads from desc_reserve:F, then
+		 * prb_first_seq:A reads from desc_push_tail:B.
+		 *
+		 * Relies on:
+		 *
+		 * MB from desc_push_tail:B to desc_reserve:F
+		 *    matching
+		 * RMB prb_first_seq:B to prb_first_seq:A
+		 */
+		smp_rmb(); /* LMM(prb_first_seq:C) */
+	}
+
+	return seq;
+}
+
+/*
+ * Non-blocking read of a record. Updates @seq to the last finalized record
+ * (which may have no data available).
+ *
+ * See the description of prb_read_valid() and prb_read_valid_info()
+ * for details.
+ */
+static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
+			    struct printk_record *r, unsigned int *line_count)
+{
+	u64 tail_seq;
+	int err;
+
+	while ((err = prb_read(rb, *seq, r, line_count))) {
+		tail_seq = prb_first_seq(rb);
+
+		if (*seq < tail_seq) {
+			/*
+			 * Behind the tail. Catch up and try again. This
+			 * can happen for -ENOENT and -EINVAL cases.
+			 */
+			*seq = tail_seq;
+
+		} else if (err == -ENOENT) {
+			/* Record exists, but no data available. Skip. */
+			(*seq)++;
+
+		} else {
+			/* Non-existent/non-finalized record. Must stop. */
+			return false;
+		}
+	}
+
+	return true;
+}
+
+/**
+ * prb_read_valid() - Non-blocking read of a requested record or (if gone)
+ *                    the next available record.
+ *
+ * @rb:  The ringbuffer to read from.
+ * @seq: The sequence number of the record to read.
+ * @r:   A record data buffer to store the read record to.
+ *
+ * This is the public function available to readers to read a record.
+ *
+ * The reader provides the @info and @text_buf buffers of @r to be
+ * filled in. Any of the buffer pointers can be set to NULL if the reader
+ * is not interested in that data. To ensure proper initialization of @r,
+ * prb_rec_init_rd() should be used.
+ *
+ * Context: Any context.
+ * Return: true if a record was read, otherwise false.
+ *
+ * On success, the reader must check r->info.seq to see which record was
+ * actually read. This allows the reader to detect dropped records.
+ *
+ * Failure means @seq refers to a not yet written record.
+ */
+bool prb_read_valid(struct printk_ringbuffer *rb, u64 seq,
+		    struct printk_record *r)
+{
+	return _prb_read_valid(rb, &seq, r, NULL);
+}
+
+/**
+ * prb_read_valid_info() - Non-blocking read of meta data for a requested
+ *                         record or (if gone) the next available record.
+ *
+ * @rb:         The ringbuffer to read from.
+ * @seq:        The sequence number of the record to read.
+ * @info:       A buffer to store the read record meta data to.
+ * @line_count: A buffer to store the number of lines in the record text.
+ *
+ * This is the public function available to readers to read only the
+ * meta data of a record.
+ *
+ * The reader provides the @info, @line_count buffers to be filled in.
+ * Either of the buffer pointers can be set to NULL if the reader is not
+ * interested in that data.
+ *
+ * Context: Any context.
+ * Return: true if a record's meta data was read, otherwise false.
+ *
+ * On success, the reader must check info->seq to see which record meta data
+ * was actually read. This allows the reader to detect dropped records.
+ *
+ * Failure means @seq refers to a not yet written record.
+ */
+bool prb_read_valid_info(struct printk_ringbuffer *rb, u64 seq,
+			 struct printk_info *info, unsigned int *line_count)
+{
+	struct printk_record r;
+
+	prb_rec_init_rd(&r, info, NULL, 0);
+
+	return _prb_read_valid(rb, &seq, &r, line_count);
+}
+
+/**
+ * prb_first_valid_seq() - Get the sequence number of the oldest available
+ *                         record.
+ *
+ * @rb: The ringbuffer to get the sequence number from.
+ *
+ * This is the public function available to readers to see what the
+ * first/oldest valid sequence number is.
+ *
+ * This provides readers a starting point to begin iterating the ringbuffer.
+ *
+ * Context: Any context.
+ * Return: The sequence number of the first/oldest record or, if the
+ *         ringbuffer is empty, 0 is returned.
+ */
+u64 prb_first_valid_seq(struct printk_ringbuffer *rb)
+{
+	u64 seq = 0;
+
+	if (!_prb_read_valid(rb, &seq, NULL, NULL))
+		return 0;
+
+	return seq;
+}
+
+/**
+ * prb_next_seq() - Get the sequence number after the last available record.
+ *
+ * @rb:  The ringbuffer to get the sequence number from.
+ *
+ * This is the public function available to readers to see what the next
+ * newest sequence number available to readers will be.
+ *
+ * This provides readers a sequence number to jump to if all currently
+ * available records should be skipped.
+ *
+ * Context: Any context.
+ * Return: The sequence number of the next newest (not yet available) record
+ *         for readers.
+ */
+u64 prb_next_seq(struct printk_ringbuffer *rb)
+{
+	u64 seq = 0;
+
+	/* Search forward from the oldest descriptor. */
+	while (_prb_read_valid(rb, &seq, NULL, NULL))
+		seq++;
+
+	return seq;
+}
+
+/**
+ * prb_init() - Initialize a ringbuffer to use provided external buffers.
+ *
+ * @rb:       The ringbuffer to initialize.
+ * @text_buf: The data buffer for text data.
+ * @textbits: The size of @text_buf as a power-of-2 value.
+ * @descs:    The descriptor buffer for ringbuffer records.
+ * @descbits: The count of @descs items as a power-of-2 value.
+ * @infos:    The printk_info buffer for ringbuffer records.
+ *
+ * This is the public function available to writers to setup a ringbuffer
+ * during runtime using provided buffers.
+ *
+ * This must match the initialization of DEFINE_PRINTKRB().
+ *
+ * Context: Any context.
+ */
+void prb_init(struct printk_ringbuffer *rb,
+	      char *text_buf, unsigned int textbits,
+	      struct prb_desc *descs, unsigned int descbits,
+	      struct printk_info *infos)
+{
+	memset(descs, 0, _DESCS_COUNT(descbits) * sizeof(descs[0]));
+	memset(infos, 0, _DESCS_COUNT(descbits) * sizeof(infos[0]));
+
+	rb->desc_ring.count_bits = descbits;
+	rb->desc_ring.descs = descs;
+	rb->desc_ring.infos = infos;
+	atomic_long_set(&rb->desc_ring.head_id, DESC0_ID(descbits));
+	atomic_long_set(&rb->desc_ring.tail_id, DESC0_ID(descbits));
+
+	rb->text_data_ring.size_bits = textbits;
+	rb->text_data_ring.data = text_buf;
+	atomic_long_set(&rb->text_data_ring.head_lpos, BLK0_LPOS(textbits));
+	atomic_long_set(&rb->text_data_ring.tail_lpos, BLK0_LPOS(textbits));
+
+	atomic_long_set(&rb->fail, 0);
+
+	atomic_long_set(&(descs[_DESCS_COUNT(descbits) - 1].state_var), DESC0_SV(descbits));
+	descs[_DESCS_COUNT(descbits) - 1].text_blk_lpos.begin = FAILED_LPOS;
+	descs[_DESCS_COUNT(descbits) - 1].text_blk_lpos.next = FAILED_LPOS;
+
+	infos[0].seq = -(u64)_DESCS_COUNT(descbits);
+	infos[_DESCS_COUNT(descbits) - 1].seq = 0;
+}
+
+/**
+ * prb_record_text_space() - Query the full actual used ringbuffer space for
+ *                           the text data of a reserved entry.
+ *
+ * @e: The successfully reserved entry to query.
+ *
+ * This is the public function available to writers to see how much actual
+ * space is used in the ringbuffer to store the text data of the specified
+ * entry.
+ *
+ * This function is only valid if @e has been successfully reserved using
+ * prb_reserve().
+ *
+ * Context: Any context.
+ * Return: The size in bytes used by the text data of the associated record.
+ */
+unsigned int prb_record_text_space(struct prb_reserved_entry *e)
+{
+	return e->text_space;
+}
diff --git a/kernel/printk/printk_ringbuffer.h b/kernel/printk/printk_ringbuffer.h
new file mode 100644
index 000000000..5dc9d022d
--- /dev/null
+++ b/kernel/printk/printk_ringbuffer.h
@@ -0,0 +1,382 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _KERNEL_PRINTK_RINGBUFFER_H
+#define _KERNEL_PRINTK_RINGBUFFER_H
+
+#include <linux/atomic.h>
+#include <linux/dev_printk.h>
+
+/*
+ * Meta information about each stored message.
+ *
+ * All fields are set by the printk code except for @seq, which is
+ * set by the ringbuffer code.
+ */
+struct printk_info {
+	u64	seq;		/* sequence number */
+	u64	ts_nsec;	/* timestamp in nanoseconds */
+	u16	text_len;	/* length of text message */
+	u8	facility;	/* syslog facility */
+	u8	flags:5;	/* internal record flags */
+	u8	level:3;	/* syslog level */
+	u32	caller_id;	/* thread id or processor id */
+
+	struct dev_printk_info	dev_info;
+};
+
+/*
+ * A structure providing the buffers, used by writers and readers.
+ *
+ * Writers:
+ * Using prb_rec_init_wr(), a writer sets @text_buf_size before calling
+ * prb_reserve(). On success, prb_reserve() sets @info and @text_buf to
+ * buffers reserved for that writer.
+ *
+ * Readers:
+ * Using prb_rec_init_rd(), a reader sets all fields before calling
+ * prb_read_valid(). Note that the reader provides the @info and @text_buf,
+ * buffers. On success, the struct pointed to by @info will be filled and
+ * the char array pointed to by @text_buf will be filled with text data.
+ */
+struct printk_record {
+	struct printk_info	*info;
+	char			*text_buf;
+	unsigned int		text_buf_size;
+};
+
+/* Specifies the logical position and span of a data block. */
+struct prb_data_blk_lpos {
+	unsigned long	begin;
+	unsigned long	next;
+};
+
+/*
+ * A descriptor: the complete meta-data for a record.
+ *
+ * @state_var: A bitwise combination of descriptor ID and descriptor state.
+ */
+struct prb_desc {
+	atomic_long_t			state_var;
+	struct prb_data_blk_lpos	text_blk_lpos;
+};
+
+/* A ringbuffer of "ID + data" elements. */
+struct prb_data_ring {
+	unsigned int	size_bits;
+	char		*data;
+	atomic_long_t	head_lpos;
+	atomic_long_t	tail_lpos;
+};
+
+/* A ringbuffer of "struct prb_desc" elements. */
+struct prb_desc_ring {
+	unsigned int		count_bits;
+	struct prb_desc		*descs;
+	struct printk_info	*infos;
+	atomic_long_t		head_id;
+	atomic_long_t		tail_id;
+};
+
+/*
+ * The high level structure representing the printk ringbuffer.
+ *
+ * @fail: Count of failed prb_reserve() calls where not even a data-less
+ *        record was created.
+ */
+struct printk_ringbuffer {
+	struct prb_desc_ring	desc_ring;
+	struct prb_data_ring	text_data_ring;
+	atomic_long_t		fail;
+};
+
+/*
+ * Used by writers as a reserve/commit handle.
+ *
+ * @rb:         Ringbuffer where the entry is reserved.
+ * @irqflags:   Saved irq flags to restore on entry commit.
+ * @id:         ID of the reserved descriptor.
+ * @text_space: Total occupied buffer space in the text data ring, including
+ *              ID, alignment padding, and wrapping data blocks.
+ *
+ * This structure is an opaque handle for writers. Its contents are only
+ * to be used by the ringbuffer implementation.
+ */
+struct prb_reserved_entry {
+	struct printk_ringbuffer	*rb;
+	unsigned long			irqflags;
+	unsigned long			id;
+	unsigned int			text_space;
+};
+
+/* The possible responses of a descriptor state-query. */
+enum desc_state {
+	desc_miss	=  -1,	/* ID mismatch (pseudo state) */
+	desc_reserved	= 0x0,	/* reserved, in use by writer */
+	desc_committed	= 0x1,	/* committed by writer, could get reopened */
+	desc_finalized	= 0x2,	/* committed, no further modification allowed */
+	desc_reusable	= 0x3,	/* free, not yet used by any writer */
+};
+
+#define _DATA_SIZE(sz_bits)	(1UL << (sz_bits))
+#define _DESCS_COUNT(ct_bits)	(1U << (ct_bits))
+#define DESC_SV_BITS		(sizeof(unsigned long) * 8)
+#define DESC_FLAGS_SHIFT	(DESC_SV_BITS - 2)
+#define DESC_FLAGS_MASK		(3UL << DESC_FLAGS_SHIFT)
+#define DESC_STATE(sv)		(3UL & (sv >> DESC_FLAGS_SHIFT))
+#define DESC_SV(id, state)	(((unsigned long)state << DESC_FLAGS_SHIFT) | id)
+#define DESC_ID_MASK		(~DESC_FLAGS_MASK)
+#define DESC_ID(sv)		((sv) & DESC_ID_MASK)
+#define FAILED_LPOS		0x1
+#define NO_LPOS			0x3
+
+#define FAILED_BLK_LPOS	\
+{				\
+	.begin	= FAILED_LPOS,	\
+	.next	= FAILED_LPOS,	\
+}
+
+/*
+ * Descriptor Bootstrap
+ *
+ * The descriptor array is minimally initialized to allow immediate usage
+ * by readers and writers. The requirements that the descriptor array
+ * initialization must satisfy:
+ *
+ *   Req1
+ *     The tail must point to an existing (committed or reusable) descriptor.
+ *     This is required by the implementation of prb_first_seq().
+ *
+ *   Req2
+ *     Readers must see that the ringbuffer is initially empty.
+ *
+ *   Req3
+ *     The first record reserved by a writer is assigned sequence number 0.
+ *
+ * To satisfy Req1, the tail initially points to a descriptor that is
+ * minimally initialized (having no data block, i.e. data-less with the
+ * data block's lpos @begin and @next values set to FAILED_LPOS).
+ *
+ * To satisfy Req2, the initial tail descriptor is initialized to the
+ * reusable state. Readers recognize reusable descriptors as existing
+ * records, but skip over them.
+ *
+ * To satisfy Req3, the last descriptor in the array is used as the initial
+ * head (and tail) descriptor. This allows the first record reserved by a
+ * writer (head + 1) to be the first descriptor in the array. (Only the first
+ * descriptor in the array could have a valid sequence number of 0.)
+ *
+ * The first time a descriptor is reserved, it is assigned a sequence number
+ * with the value of the array index. A "first time reserved" descriptor can
+ * be recognized because it has a sequence number of 0 but does not have an
+ * index of 0. (Only the first descriptor in the array could have a valid
+ * sequence number of 0.) After the first reservation, all future reservations
+ * (recycling) simply involve incrementing the sequence number by the array
+ * count.
+ *
+ *   Hack #1
+ *     Only the first descriptor in the array is allowed to have the sequence
+ *     number 0. In this case it is not possible to recognize if it is being
+ *     reserved the first time (set to index value) or has been reserved
+ *     previously (increment by the array count). This is handled by _always_
+ *     incrementing the sequence number by the array count when reserving the
+ *     first descriptor in the array. In order to satisfy Req3, the sequence
+ *     number of the first descriptor in the array is initialized to minus
+ *     the array count. Then, upon the first reservation, it is incremented
+ *     to 0, thus satisfying Req3.
+ *
+ *   Hack #2
+ *     prb_first_seq() can be called at any time by readers to retrieve the
+ *     sequence number of the tail descriptor. However, due to Req2 and Req3,
+ *     initially there are no records to report the sequence number of
+ *     (sequence numbers are u64 and there is nothing less than 0). To handle
+ *     this, the sequence number of the initial tail descriptor is initialized
+ *     to 0. Technically this is incorrect, because there is no record with
+ *     sequence number 0 (yet) and the tail descriptor is not the first
+ *     descriptor in the array. But it allows prb_read_valid() to correctly
+ *     report the existence of a record for _any_ given sequence number at all
+ *     times. Bootstrapping is complete when the tail is pushed the first
+ *     time, thus finally pointing to the first descriptor reserved by a
+ *     writer, which has the assigned sequence number 0.
+ */
+
+/*
+ * Initiating Logical Value Overflows
+ *
+ * Both logical position (lpos) and ID values can be mapped to array indexes
+ * but may experience overflows during the lifetime of the system. To ensure
+ * that printk_ringbuffer can handle the overflows for these types, initial
+ * values are chosen that map to the correct initial array indexes, but will
+ * result in overflows soon.
+ *
+ *   BLK0_LPOS
+ *     The initial @head_lpos and @tail_lpos for data rings. It is at index
+ *     0 and the lpos value is such that it will overflow on the first wrap.
+ *
+ *   DESC0_ID
+ *     The initial @head_id and @tail_id for the desc ring. It is at the last
+ *     index of the descriptor array (see Req3 above) and the ID value is such
+ *     that it will overflow on the second wrap.
+ */
+#define BLK0_LPOS(sz_bits)	(-(_DATA_SIZE(sz_bits)))
+#define DESC0_ID(ct_bits)	DESC_ID(-(_DESCS_COUNT(ct_bits) + 1))
+#define DESC0_SV(ct_bits)	DESC_SV(DESC0_ID(ct_bits), desc_reusable)
+
+/*
+ * Define a ringbuffer with an external text data buffer. The same as
+ * DEFINE_PRINTKRB() but requires specifying an external buffer for the
+ * text data.
+ *
+ * Note: The specified external buffer must be of the size:
+ *       2 ^ (descbits + avgtextbits)
+ */
+#define _DEFINE_PRINTKRB(name, descbits, avgtextbits, text_buf)			\
+static struct prb_desc _##name##_descs[_DESCS_COUNT(descbits)] = {				\
+	/* the initial head and tail */								\
+	[_DESCS_COUNT(descbits) - 1] = {							\
+		/* reusable */									\
+		.state_var	= ATOMIC_INIT(DESC0_SV(descbits)),				\
+		/* no associated data block */							\
+		.text_blk_lpos	= FAILED_BLK_LPOS,						\
+	},											\
+};												\
+static struct printk_info _##name##_infos[_DESCS_COUNT(descbits)] = {				\
+	/* this will be the first record reserved by a writer */				\
+	[0] = {											\
+		/* will be incremented to 0 on the first reservation */				\
+		.seq = -(u64)_DESCS_COUNT(descbits),						\
+	},											\
+	/* the initial head and tail */								\
+	[_DESCS_COUNT(descbits) - 1] = {							\
+		/* reports the first seq value during the bootstrap phase */			\
+		.seq = 0,									\
+	},											\
+};												\
+static struct printk_ringbuffer name = {							\
+	.desc_ring = {										\
+		.count_bits	= descbits,							\
+		.descs		= &_##name##_descs[0],						\
+		.infos		= &_##name##_infos[0],						\
+		.head_id	= ATOMIC_INIT(DESC0_ID(descbits)),				\
+		.tail_id	= ATOMIC_INIT(DESC0_ID(descbits)),				\
+	},											\
+	.text_data_ring = {									\
+		.size_bits	= (avgtextbits) + (descbits),					\
+		.data		= text_buf,							\
+		.head_lpos	= ATOMIC_LONG_INIT(BLK0_LPOS((avgtextbits) + (descbits))),	\
+		.tail_lpos	= ATOMIC_LONG_INIT(BLK0_LPOS((avgtextbits) + (descbits))),	\
+	},											\
+	.fail			= ATOMIC_LONG_INIT(0),						\
+}
+
+/**
+ * DEFINE_PRINTKRB() - Define a ringbuffer.
+ *
+ * @name:        The name of the ringbuffer variable.
+ * @descbits:    The number of descriptors as a power-of-2 value.
+ * @avgtextbits: The average text data size per record as a power-of-2 value.
+ *
+ * This is a macro for defining a ringbuffer and all internal structures
+ * such that it is ready for immediate use. See _DEFINE_PRINTKRB() for a
+ * variant where the text data buffer can be specified externally.
+ */
+#define DEFINE_PRINTKRB(name, descbits, avgtextbits)				\
+static char _##name##_text[1U << ((avgtextbits) + (descbits))]			\
+			__aligned(__alignof__(unsigned long));			\
+_DEFINE_PRINTKRB(name, descbits, avgtextbits, &_##name##_text[0])
+
+/* Writer Interface */
+
+/**
+ * prb_rec_init_wd() - Initialize a buffer for writing records.
+ *
+ * @r:             The record to initialize.
+ * @text_buf_size: The needed text buffer size.
+ */
+static inline void prb_rec_init_wr(struct printk_record *r,
+				   unsigned int text_buf_size)
+{
+	r->info = NULL;
+	r->text_buf = NULL;
+	r->text_buf_size = text_buf_size;
+}
+
+bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
+		 struct printk_record *r);
+bool prb_reserve_in_last(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
+			 struct printk_record *r, u32 caller_id, unsigned int max_size);
+void prb_commit(struct prb_reserved_entry *e);
+void prb_final_commit(struct prb_reserved_entry *e);
+
+void prb_init(struct printk_ringbuffer *rb,
+	      char *text_buf, unsigned int text_buf_size,
+	      struct prb_desc *descs, unsigned int descs_count_bits,
+	      struct printk_info *infos);
+unsigned int prb_record_text_space(struct prb_reserved_entry *e);
+
+/* Reader Interface */
+
+/**
+ * prb_rec_init_rd() - Initialize a buffer for reading records.
+ *
+ * @r:             The record to initialize.
+ * @info:          A buffer to store record meta-data.
+ * @text_buf:      A buffer to store text data.
+ * @text_buf_size: The size of @text_buf.
+ *
+ * Initialize all the fields that a reader is interested in. All arguments
+ * (except @r) are optional. Only record data for arguments that are
+ * non-NULL or non-zero will be read.
+ */
+static inline void prb_rec_init_rd(struct printk_record *r,
+				   struct printk_info *info,
+				   char *text_buf, unsigned int text_buf_size)
+{
+	r->info = info;
+	r->text_buf = text_buf;
+	r->text_buf_size = text_buf_size;
+}
+
+/**
+ * prb_for_each_record() - Iterate over the records of a ringbuffer.
+ *
+ * @from: The sequence number to begin with.
+ * @rb:   The ringbuffer to iterate over.
+ * @s:    A u64 to store the sequence number on each iteration.
+ * @r:    A printk_record to store the record on each iteration.
+ *
+ * This is a macro for conveniently iterating over a ringbuffer.
+ * Note that @s may not be the sequence number of the record on each
+ * iteration. For the sequence number, @r->info->seq should be checked.
+ *
+ * Context: Any context.
+ */
+#define prb_for_each_record(from, rb, s, r) \
+for ((s) = from; prb_read_valid(rb, s, r); (s) = (r)->info->seq + 1)
+
+/**
+ * prb_for_each_info() - Iterate over the meta data of a ringbuffer.
+ *
+ * @from: The sequence number to begin with.
+ * @rb:   The ringbuffer to iterate over.
+ * @s:    A u64 to store the sequence number on each iteration.
+ * @i:    A printk_info to store the record meta data on each iteration.
+ * @lc:   An unsigned int to store the text line count of each record.
+ *
+ * This is a macro for conveniently iterating over a ringbuffer.
+ * Note that @s may not be the sequence number of the record on each
+ * iteration. For the sequence number, @r->info->seq should be checked.
+ *
+ * Context: Any context.
+ */
+#define prb_for_each_info(from, rb, s, i, lc) \
+for ((s) = from; prb_read_valid_info(rb, s, i, lc); (s) = (i)->seq + 1)
+
+bool prb_read_valid(struct printk_ringbuffer *rb, u64 seq,
+		    struct printk_record *r);
+bool prb_read_valid_info(struct printk_ringbuffer *rb, u64 seq,
+			 struct printk_info *info, unsigned int *line_count);
+
+u64 prb_first_valid_seq(struct printk_ringbuffer *rb);
+u64 prb_next_seq(struct printk_ringbuffer *rb);
+
+#endif /* _KERNEL_PRINTK_RINGBUFFER_H */
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
new file mode 100644
index 000000000..2e9e3ed7d
--- /dev/null
+++ b/kernel/printk/printk_safe.c
@@ -0,0 +1,422 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * printk_safe.c - Safe printk for printk-deadlock-prone contexts
+ */
+
+#include <linux/preempt.h>
+#include <linux/spinlock.h>
+#include <linux/debug_locks.h>
+#include <linux/kdb.h>
+#include <linux/smp.h>
+#include <linux/cpumask.h>
+#include <linux/irq_work.h>
+#include <linux/printk.h>
+#include <linux/kprobes.h>
+
+#include "internal.h"
+
+/*
+ * printk() could not take logbuf_lock in NMI context. Instead,
+ * it uses an alternative implementation that temporary stores
+ * the strings into a per-CPU buffer. The content of the buffer
+ * is later flushed into the main ring buffer via IRQ work.
+ *
+ * The alternative implementation is chosen transparently
+ * by examining current printk() context mask stored in @printk_context
+ * per-CPU variable.
+ *
+ * The implementation allows to flush the strings also from another CPU.
+ * There are situations when we want to make sure that all buffers
+ * were handled or when IRQs are blocked.
+ */
+
+#define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) -	\
+				sizeof(atomic_t) -			\
+				sizeof(atomic_t) -			\
+				sizeof(struct irq_work))
+
+struct printk_safe_seq_buf {
+	atomic_t		len;	/* length of written data */
+	atomic_t		message_lost;
+	struct irq_work		work;	/* IRQ work that flushes the buffer */
+	unsigned char		buffer[SAFE_LOG_BUF_LEN];
+};
+
+static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq);
+static DEFINE_PER_CPU(int, printk_context);
+
+static DEFINE_RAW_SPINLOCK(safe_read_lock);
+
+#ifdef CONFIG_PRINTK_NMI
+static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
+#endif
+
+/* Get flushed in a more safe context. */
+static void queue_flush_work(struct printk_safe_seq_buf *s)
+{
+	if (printk_percpu_data_ready())
+		irq_work_queue(&s->work);
+}
+
+/*
+ * Add a message to per-CPU context-dependent buffer. NMI and printk-safe
+ * have dedicated buffers, because otherwise printk-safe preempted by
+ * NMI-printk would have overwritten the NMI messages.
+ *
+ * The messages are flushed from irq work (or from panic()), possibly,
+ * from other CPU, concurrently with printk_safe_log_store(). Should this
+ * happen, printk_safe_log_store() will notice the buffer->len mismatch
+ * and repeat the write.
+ */
+static __printf(2, 0) int printk_safe_log_store(struct printk_safe_seq_buf *s,
+						const char *fmt, va_list args)
+{
+	int add;
+	size_t len;
+	va_list ap;
+
+again:
+	len = atomic_read(&s->len);
+
+	/* The trailing '\0' is not counted into len. */
+	if (len >= sizeof(s->buffer) - 1) {
+		atomic_inc(&s->message_lost);
+		queue_flush_work(s);
+		return 0;
+	}
+
+	/*
+	 * Make sure that all old data have been read before the buffer
+	 * was reset. This is not needed when we just append data.
+	 */
+	if (!len)
+		smp_rmb();
+
+	va_copy(ap, args);
+	add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, ap);
+	va_end(ap);
+	if (!add)
+		return 0;
+
+	/*
+	 * Do it once again if the buffer has been flushed in the meantime.
+	 * Note that atomic_cmpxchg() is an implicit memory barrier that
+	 * makes sure that the data were written before updating s->len.
+	 */
+	if (atomic_cmpxchg(&s->len, len, len + add) != len)
+		goto again;
+
+	queue_flush_work(s);
+	return add;
+}
+
+static inline void printk_safe_flush_line(const char *text, int len)
+{
+	/*
+	 * Avoid any console drivers calls from here, because we may be
+	 * in NMI or printk_safe context (when in panic). The messages
+	 * must go only into the ring buffer at this stage.  Consoles will
+	 * get explicitly called later when a crashdump is not generated.
+	 */
+	printk_deferred("%.*s", len, text);
+}
+
+/* printk part of the temporary buffer line by line */
+static int printk_safe_flush_buffer(const char *start, size_t len)
+{
+	const char *c, *end;
+	bool header;
+
+	c = start;
+	end = start + len;
+	header = true;
+
+	/* Print line by line. */
+	while (c < end) {
+		if (*c == '\n') {
+			printk_safe_flush_line(start, c - start + 1);
+			start = ++c;
+			header = true;
+			continue;
+		}
+
+		/* Handle continuous lines or missing new line. */
+		if ((c + 1 < end) && printk_get_level(c)) {
+			if (header) {
+				c = printk_skip_level(c);
+				continue;
+			}
+
+			printk_safe_flush_line(start, c - start);
+			start = c++;
+			header = true;
+			continue;
+		}
+
+		header = false;
+		c++;
+	}
+
+	/* Check if there was a partial line. Ignore pure header. */
+	if (start < end && !header) {
+		static const char newline[] = KERN_CONT "\n";
+
+		printk_safe_flush_line(start, end - start);
+		printk_safe_flush_line(newline, strlen(newline));
+	}
+
+	return len;
+}
+
+static void report_message_lost(struct printk_safe_seq_buf *s)
+{
+	int lost = atomic_xchg(&s->message_lost, 0);
+
+	if (lost)
+		printk_deferred("Lost %d message(s)!\n", lost);
+}
+
+/*
+ * Flush data from the associated per-CPU buffer. The function
+ * can be called either via IRQ work or independently.
+ */
+static void __printk_safe_flush(struct irq_work *work)
+{
+	struct printk_safe_seq_buf *s =
+		container_of(work, struct printk_safe_seq_buf, work);
+	unsigned long flags;
+	size_t len;
+	int i;
+
+	/*
+	 * The lock has two functions. First, one reader has to flush all
+	 * available message to make the lockless synchronization with
+	 * writers easier. Second, we do not want to mix messages from
+	 * different CPUs. This is especially important when printing
+	 * a backtrace.
+	 */
+	raw_spin_lock_irqsave(&safe_read_lock, flags);
+
+	i = 0;
+more:
+	len = atomic_read(&s->len);
+
+	/*
+	 * This is just a paranoid check that nobody has manipulated
+	 * the buffer an unexpected way. If we printed something then
+	 * @len must only increase. Also it should never overflow the
+	 * buffer size.
+	 */
+	if ((i && i >= len) || len > sizeof(s->buffer)) {
+		const char *msg = "printk_safe_flush: internal error\n";
+
+		printk_safe_flush_line(msg, strlen(msg));
+		len = 0;
+	}
+
+	if (!len)
+		goto out; /* Someone else has already flushed the buffer. */
+
+	/* Make sure that data has been written up to the @len */
+	smp_rmb();
+	i += printk_safe_flush_buffer(s->buffer + i, len - i);
+
+	/*
+	 * Check that nothing has got added in the meantime and truncate
+	 * the buffer. Note that atomic_cmpxchg() is an implicit memory
+	 * barrier that makes sure that the data were copied before
+	 * updating s->len.
+	 */
+	if (atomic_cmpxchg(&s->len, len, 0) != len)
+		goto more;
+
+out:
+	report_message_lost(s);
+	raw_spin_unlock_irqrestore(&safe_read_lock, flags);
+}
+
+/**
+ * printk_safe_flush - flush all per-cpu nmi buffers.
+ *
+ * The buffers are flushed automatically via IRQ work. This function
+ * is useful only when someone wants to be sure that all buffers have
+ * been flushed at some point.
+ */
+void printk_safe_flush(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+#ifdef CONFIG_PRINTK_NMI
+		__printk_safe_flush(&per_cpu(nmi_print_seq, cpu).work);
+#endif
+		__printk_safe_flush(&per_cpu(safe_print_seq, cpu).work);
+	}
+}
+
+/**
+ * printk_safe_flush_on_panic - flush all per-cpu nmi buffers when the system
+ *	goes down.
+ *
+ * Similar to printk_safe_flush() but it can be called even in NMI context when
+ * the system goes down. It does the best effort to get NMI messages into
+ * the main ring buffer.
+ *
+ * Note that it could try harder when there is only one CPU online.
+ */
+void printk_safe_flush_on_panic(void)
+{
+	/*
+	 * Make sure that we could access the main ring buffer.
+	 * Do not risk a double release when more CPUs are up.
+	 */
+	if (raw_spin_is_locked(&logbuf_lock)) {
+		if (num_online_cpus() > 1)
+			return;
+
+		debug_locks_off();
+		raw_spin_lock_init(&logbuf_lock);
+	}
+
+	if (raw_spin_is_locked(&safe_read_lock)) {
+		if (num_online_cpus() > 1)
+			return;
+
+		debug_locks_off();
+		raw_spin_lock_init(&safe_read_lock);
+	}
+
+	printk_safe_flush();
+}
+
+#ifdef CONFIG_PRINTK_NMI
+/*
+ * Safe printk() for NMI context. It uses a per-CPU buffer to
+ * store the message. NMIs are not nested, so there is always only
+ * one writer running. But the buffer might get flushed from another
+ * CPU, so we need to be careful.
+ */
+static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
+{
+	struct printk_safe_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
+
+	return printk_safe_log_store(s, fmt, args);
+}
+
+void noinstr printk_nmi_enter(void)
+{
+	this_cpu_add(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
+}
+
+void noinstr printk_nmi_exit(void)
+{
+	this_cpu_sub(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
+}
+
+/*
+ * Marks a code that might produce many messages in NMI context
+ * and the risk of losing them is more critical than eventual
+ * reordering.
+ *
+ * It has effect only when called in NMI context. Then printk()
+ * will try to store the messages into the main logbuf directly
+ * and use the per-CPU buffers only as a fallback when the lock
+ * is not available.
+ */
+void printk_nmi_direct_enter(void)
+{
+	if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
+		this_cpu_or(printk_context, PRINTK_NMI_DIRECT_CONTEXT_MASK);
+}
+
+void printk_nmi_direct_exit(void)
+{
+	this_cpu_and(printk_context, ~PRINTK_NMI_DIRECT_CONTEXT_MASK);
+}
+
+#else
+
+static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
+{
+	return 0;
+}
+
+#endif /* CONFIG_PRINTK_NMI */
+
+/*
+ * Lock-less printk(), to avoid deadlocks should the printk() recurse
+ * into itself. It uses a per-CPU buffer to store the message, just like
+ * NMI.
+ */
+static __printf(1, 0) int vprintk_safe(const char *fmt, va_list args)
+{
+	struct printk_safe_seq_buf *s = this_cpu_ptr(&safe_print_seq);
+
+	return printk_safe_log_store(s, fmt, args);
+}
+
+/* Can be preempted by NMI. */
+void __printk_safe_enter(void)
+{
+	this_cpu_inc(printk_context);
+}
+
+/* Can be preempted by NMI. */
+void __printk_safe_exit(void)
+{
+	this_cpu_dec(printk_context);
+}
+
+__printf(1, 0) int vprintk_func(const char *fmt, va_list args)
+{
+#ifdef CONFIG_KGDB_KDB
+	/* Allow to pass printk() to kdb but avoid a recursion. */
+	if (unlikely(kdb_trap_printk && kdb_printf_cpu < 0))
+		return vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
+#endif
+
+	/*
+	 * Try to use the main logbuf even in NMI. But avoid calling console
+	 * drivers that might have their own locks.
+	 */
+	if ((this_cpu_read(printk_context) & PRINTK_NMI_DIRECT_CONTEXT_MASK) &&
+	    raw_spin_trylock(&logbuf_lock)) {
+		int len;
+
+		len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
+		raw_spin_unlock(&logbuf_lock);
+		defer_console_output();
+		return len;
+	}
+
+	/* Use extra buffer in NMI when logbuf_lock is taken or in safe mode. */
+	if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
+		return vprintk_nmi(fmt, args);
+
+	/* Use extra buffer to prevent a recursion deadlock in safe mode. */
+	if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK)
+		return vprintk_safe(fmt, args);
+
+	/* No obstacles. */
+	return vprintk_default(fmt, args);
+}
+
+void __init printk_safe_init(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		struct printk_safe_seq_buf *s;
+
+		s = &per_cpu(safe_print_seq, cpu);
+		init_irq_work(&s->work, __printk_safe_flush);
+
+#ifdef CONFIG_PRINTK_NMI
+		s = &per_cpu(nmi_print_seq, cpu);
+		init_irq_work(&s->work, __printk_safe_flush);
+#endif
+	}
+
+	/* Flush pending messages that did not have scheduled IRQ works. */
+	printk_safe_flush();
+}