From dd4e52ae061806da556008ca819481befcff0fd3 Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Tue, 19 Jul 2022 20:08:01 +0200 Subject: [PATCH 19/62] printk: Bring back the RT bits. Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.1/older/patches-6.1.69-rt21.tar.xz This is a revert of the commits: | 07a22b61946f0 Revert "printk: add functions to prefer direct printing" | 5831788afb17b Revert "printk: add kthread console printers" | 2d9ef940f89e0 Revert "printk: extend console_lock for per-console locking" | 007eeab7e9f03 Revert "printk: remove @console_locked" | 05c96b3713aa2 Revert "printk: Block console kthreads when direct printing will be required" | 20fb0c8272bbb Revert "printk: Wait for the global console lock when the system is going down" which is needed for the atomic consoles which are used on PREEMPT_RT. Signed-off-by: Sebastian Andrzej Siewior --- drivers/tty/sysrq.c | 2 + include/linux/console.h | 17 ++ include/linux/printk.h | 15 + kernel/hung_task.c | 11 +- kernel/panic.c | 4 + kernel/printk/internal.h | 2 + kernel/printk/printk.c | 586 ++++++++++++++++++++++++++++++++---- kernel/printk/printk_safe.c | 32 ++ kernel/rcu/tree_stall.h | 2 + kernel/reboot.c | 16 +- kernel/watchdog.c | 4 + kernel/watchdog_hld.c | 4 + 12 files changed, 636 insertions(+), 59 deletions(-) diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c index 248067197287..0db9dad8c99f 100644 --- a/drivers/tty/sysrq.c +++ b/drivers/tty/sysrq.c @@ -582,6 +582,7 @@ void __handle_sysrq(int key, bool check_mask) rcu_sysrq_start(); rcu_read_lock(); + printk_prefer_direct_enter(); /* * Raise the apparent loglevel to maximum so that the sysrq header * is shown to provide the user with positive feedback. We do not @@ -623,6 +624,7 @@ void __handle_sysrq(int key, bool check_mask) pr_cont("\n"); console_loglevel = orig_log_level; } + printk_prefer_direct_exit(); rcu_read_unlock(); rcu_sysrq_end(); diff --git a/include/linux/console.h b/include/linux/console.h index 8c1686e2c233..143653090c48 100644 --- a/include/linux/console.h +++ b/include/linux/console.h @@ -16,6 +16,7 @@ #include #include +#include struct vc_data; struct console_font_op; @@ -153,6 +154,22 @@ struct console { uint ospeed; u64 seq; unsigned long dropped; + struct task_struct *thread; + bool blocked; + + /* + * The per-console lock is used by printing kthreads to synchronize + * this console with callers of console_lock(). This is necessary in + * order to allow printing kthreads to run in parallel to each other, + * while each safely accessing the @blocked field and synchronizing + * against direct printing via console_lock/console_unlock. + * + * Note: For synchronizing against direct printing via + * console_trylock/console_unlock, see the static global + * variable @console_kthreads_active. + */ + struct mutex lock; + void *data; struct console *next; }; diff --git a/include/linux/printk.h b/include/linux/printk.h index 8c81806c2e99..f8c4e4fa6d7d 100644 --- a/include/linux/printk.h +++ b/include/linux/printk.h @@ -168,6 +168,9 @@ extern void __printk_safe_exit(void); */ #define printk_deferred_enter __printk_safe_enter #define printk_deferred_exit __printk_safe_exit +extern void printk_prefer_direct_enter(void); +extern void printk_prefer_direct_exit(void); +extern void try_block_console_kthreads(int timeout_ms); /* * Please don't use printk_ratelimit(), because it shares ratelimiting state @@ -219,6 +222,18 @@ static inline void printk_deferred_exit(void) { } +static inline void printk_prefer_direct_enter(void) +{ +} + +static inline void printk_prefer_direct_exit(void) +{ +} + +static inline void try_block_console_kthreads(int timeout_ms) +{ +} + static inline int printk_ratelimit(void) { return 0; diff --git a/kernel/hung_task.c b/kernel/hung_task.c index c71889f3f3fc..e2d2344cb9f4 100644 --- a/kernel/hung_task.c +++ b/kernel/hung_task.c @@ -127,6 +127,8 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout) * complain: */ if (sysctl_hung_task_warnings) { + printk_prefer_direct_enter(); + if (sysctl_hung_task_warnings > 0) sysctl_hung_task_warnings--; pr_err("INFO: task %s:%d blocked for more than %ld seconds.\n", @@ -142,6 +144,8 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout) if (sysctl_hung_task_all_cpu_backtrace) hung_task_show_all_bt = true; + + printk_prefer_direct_exit(); } touch_nmi_watchdog(); @@ -212,12 +216,17 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout) } unlock: rcu_read_unlock(); - if (hung_task_show_lock) + if (hung_task_show_lock) { + printk_prefer_direct_enter(); debug_show_all_locks(); + printk_prefer_direct_exit(); + } if (hung_task_show_all_bt) { hung_task_show_all_bt = false; + printk_prefer_direct_enter(); trigger_all_cpu_backtrace(); + printk_prefer_direct_exit(); } if (hung_task_call_panic) diff --git a/kernel/panic.c b/kernel/panic.c index 63e94f3bd8dc..88cd873c7c30 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -653,6 +653,8 @@ void __warn(const char *file, int line, void *caller, unsigned taint, { disable_trace_on_warning(); + printk_prefer_direct_enter(); + if (file) pr_warn("WARNING: CPU: %d PID: %d at %s:%d %pS\n", raw_smp_processor_id(), current->pid, file, line, @@ -681,6 +683,8 @@ void __warn(const char *file, int line, void *caller, unsigned taint, /* Just a warning, don't kill lockdep. */ add_taint(taint, LOCKDEP_STILL_OK); + + printk_prefer_direct_exit(); } #ifndef __WARN_FLAGS diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h index d947ca6c84f9..e7d8578860ad 100644 --- a/kernel/printk/internal.h +++ b/kernel/printk/internal.h @@ -20,6 +20,8 @@ enum printk_info_flags { LOG_CONT = 8, /* text is a fragment of a continuation line */ }; +extern bool block_console_kthreads; + __printf(4, 0) int vprintk_store(int facility, int level, const struct dev_printk_info *dev_info, diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index cc53fb77f77c..e9f9b66608a0 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -220,6 +220,36 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write, } #endif /* CONFIG_PRINTK && CONFIG_SYSCTL */ +/* + * Used to synchronize printing kthreads against direct printing via + * console_trylock/console_unlock. + * + * Values: + * -1 = console kthreads atomically blocked (via global trylock) + * 0 = no kthread printing, console not locked (via trylock) + * >0 = kthread(s) actively printing + * + * Note: For synchronizing against direct printing via + * console_lock/console_unlock, see the @lock variable in + * struct console. + */ +static atomic_t console_kthreads_active = ATOMIC_INIT(0); + +#define console_kthreads_atomic_tryblock() \ + (atomic_cmpxchg(&console_kthreads_active, 0, -1) == 0) +#define console_kthreads_atomic_unblock() \ + atomic_cmpxchg(&console_kthreads_active, -1, 0) +#define console_kthreads_atomically_blocked() \ + (atomic_read(&console_kthreads_active) == -1) + +#define console_kthread_printing_tryenter() \ + atomic_inc_unless_negative(&console_kthreads_active) +#define console_kthread_printing_exit() \ + atomic_dec(&console_kthreads_active) + +/* Block console kthreads to avoid processing new messages. */ +bool block_console_kthreads; + /* * Helper macros to handle lockdep when locking/unlocking console_sem. We use * macros instead of functions so that _RET_IP_ contains useful information. @@ -268,14 +298,49 @@ static bool panic_in_progress(void) } /* - * 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 semaphore held). + * Tracks whether kthread printers are all blocked. A value of true implies + * that the console is locked via console_lock() or the console is suspended. + * Writing to this variable requires holding @console_sem. + */ +static bool console_kthreads_blocked; + +/* + * Block all kthread printers from a schedulable context. + * + * Requires holding @console_sem. + */ +static void console_kthreads_block(void) +{ + struct console *con; + + for_each_console(con) { + mutex_lock(&con->lock); + con->blocked = true; + mutex_unlock(&con->lock); + } + + console_kthreads_blocked = true; +} + +/* + * Unblock all kthread printers from a schedulable context. + * + * Requires holding @console_sem. */ -static int console_locked, console_suspended; +static void console_kthreads_unblock(void) +{ + struct console *con; + + for_each_console(con) { + mutex_lock(&con->lock); + con->blocked = false; + mutex_unlock(&con->lock); + } + + console_kthreads_blocked = false; +} + +static int console_suspended; /* * Array of consoles built from command line options (console=) @@ -358,7 +423,75 @@ static int console_msg_format = MSG_FORMAT_DEFAULT; /* syslog_lock protects syslog_* variables and write access to clear_seq. */ static DEFINE_MUTEX(syslog_lock); +/* + * A flag to signify if printk_activate_kthreads() has already started the + * kthread printers. If true, any later registered consoles must start their + * own kthread directly. The flag is write protected by the console_lock. + */ +static bool printk_kthreads_available; + #ifdef CONFIG_PRINTK +static atomic_t printk_prefer_direct = ATOMIC_INIT(0); + +/** + * printk_prefer_direct_enter - cause printk() calls to attempt direct + * printing to all enabled consoles + * + * Since it is not possible to call into the console printing code from any + * context, there is no guarantee that direct printing will occur. + * + * This globally effects all printk() callers. + * + * Context: Any context. + */ +void printk_prefer_direct_enter(void) +{ + atomic_inc(&printk_prefer_direct); +} + +/** + * printk_prefer_direct_exit - restore printk() behavior + * + * Context: Any context. + */ +void printk_prefer_direct_exit(void) +{ + WARN_ON(atomic_dec_if_positive(&printk_prefer_direct) < 0); +} + +/* + * Calling printk() always wakes kthread printers so that they can + * flush the new message to their respective consoles. Also, if direct + * printing is allowed, printk() tries to flush the messages directly. + * + * Direct printing is allowed in situations when the kthreads + * are not available or the system is in a problematic state. + * + * See the implementation about possible races. + */ +static inline bool allow_direct_printing(void) +{ + /* + * Checking kthread availability is a possible race because the + * kthread printers can become permanently disabled during runtime. + * However, doing that requires holding the console_lock, so any + * pending messages will be direct printed by console_unlock(). + */ + if (!printk_kthreads_available) + return true; + + /* + * Prefer direct printing when the system is in a problematic state. + * The context that sets this state will always see the updated value. + * The other contexts do not care. Anyway, direct printing is just a + * best effort. The direct output is only possible when console_lock + * is not already taken and no kthread printers are actively printing. + */ + return (system_state > SYSTEM_RUNNING || + oops_in_progress || + atomic_read(&printk_prefer_direct)); +} + DECLARE_WAIT_QUEUE_HEAD(log_wait); /* All 3 protected by @syslog_lock. */ /* the next printk record to read by syslog(READ) or /proc/kmsg */ @@ -2249,10 +2382,10 @@ asmlinkage int vprintk_emit(int facility, int level, printed_len = vprintk_store(facility, level, dev_info, fmt, args); /* If called from the scheduler, we can not call up(). */ - if (!in_sched) { + if (!in_sched && allow_direct_printing()) { /* * The caller may be holding system-critical or - * timing-sensitive locks. Disable preemption during + * timing-sensitive locks. Disable preemption during direct * printing of all remaining records to all consoles so that * this context can return as soon as possible. Hopefully * another printk() caller will take over the printing. @@ -2300,6 +2433,8 @@ EXPORT_SYMBOL(_printk); static bool pr_flush(int timeout_ms, bool reset_on_progress); static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress); +static void printk_start_kthread(struct console *con); + #else /* CONFIG_PRINTK */ #define CONSOLE_LOG_MAX 0 @@ -2334,6 +2469,8 @@ static void call_console_driver(struct console *con, const char *text, size_t le static bool suppress_message_printing(int level) { return false; } static bool pr_flush(int timeout_ms, bool reset_on_progress) { return true; } static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress) { return true; } +static void printk_start_kthread(struct console *con) { } +static bool allow_direct_printing(void) { return true; } #endif /* CONFIG_PRINTK */ @@ -2552,6 +2689,14 @@ static int console_cpu_notify(unsigned int cpu) /* If trylock fails, someone else is doing the printing */ if (console_trylock()) console_unlock(); + else { + /* + * If a new CPU comes online, the conditions for + * printer_should_wake() may have changed for some + * kthread printer with !CON_ANYTIME. + */ + wake_up_klogd(); + } } return 0; } @@ -2594,7 +2739,7 @@ void console_lock(void) down_console_sem(); if (console_suspended) return; - console_locked = 1; + console_kthreads_block(); console_may_schedule = 1; } EXPORT_SYMBOL(console_lock); @@ -2618,15 +2763,30 @@ int console_trylock(void) up_console_sem(); return 0; } - console_locked = 1; + if (!console_kthreads_atomic_tryblock()) { + up_console_sem(); + return 0; + } console_may_schedule = 0; return 1; } EXPORT_SYMBOL(console_trylock); +/* + * This is used to help to make sure that certain paths within the VT code are + * running with the console lock held. It is definitely not the perfect debug + * tool (it is not known if the VT code is the task holding the console lock), + * but it helps tracking those weird code paths in the console code such as + * when the console is suspended: where the console is not locked but no + * console printing may occur. + * + * Note: This returns true when the console is suspended but is not locked. + * This is intentional because the VT code must consider that situation + * the same as if the console was locked. + */ int is_console_locked(void) { - return console_locked; + return (console_kthreads_blocked || atomic_read(&console_kthreads_active)); } EXPORT_SYMBOL(is_console_locked); @@ -2636,12 +2796,9 @@ EXPORT_SYMBOL(is_console_locked); * * Requires the console_lock. */ -static inline bool console_is_usable(struct console *con) +static inline bool __console_is_usable(short flags) { - if (!(con->flags & CON_ENABLED)) - return false; - - if (!con->write) + if (!(flags & CON_ENABLED)) return false; /* @@ -2650,15 +2807,43 @@ static inline bool console_is_usable(struct console *con) * cope (CON_ANYTIME) don't call them until this CPU is officially up. */ if (!cpu_online(raw_smp_processor_id()) && - !(con->flags & CON_ANYTIME)) + !(flags & CON_ANYTIME)) return false; return true; } +/* + * Check if the given console is currently capable and allowed to print + * records. + * + * Requires holding the console_lock. + */ +static inline bool console_is_usable(struct console *con) +{ + if (!con->write) + return false; + + return __console_is_usable(con->flags); +} + static void __console_unlock(void) { - console_locked = 0; + /* + * Depending on whether console_lock() or console_trylock() was used, + * appropriately allow the kthread printers to continue. + */ + if (console_kthreads_blocked) + console_kthreads_unblock(); + else + console_kthreads_atomic_unblock(); + + /* + * New records may have arrived while the console was locked. + * Wake the kthread printers to print them. + */ + wake_up_klogd(); + up_console_sem(); } @@ -2676,17 +2861,19 @@ static void __console_unlock(void) * * @handover will be set to true if a printk waiter has taken over the * console_lock, in which case the caller is no longer holding the - * console_lock. Otherwise it is set to false. + * console_lock. Otherwise it is set to false. A NULL pointer may be provided + * to disable allowing the console_lock to be taken over by a printk waiter. * * Returns false if the given console has no next record to print, otherwise * true. * - * Requires the console_lock. + * Requires the console_lock if @handover is non-NULL. + * Requires con->lock otherwise. */ -static bool console_emit_next_record(struct console *con, char *text, char *ext_text, - char *dropped_text, bool *handover) +static bool __console_emit_next_record(struct console *con, char *text, char *ext_text, + char *dropped_text, bool *handover) { - static int panic_console_dropped; + static atomic_t panic_console_dropped = ATOMIC_INIT(0); struct printk_info info; struct printk_record r; unsigned long flags; @@ -2695,7 +2882,8 @@ static bool console_emit_next_record(struct console *con, char *text, char *ext_ prb_rec_init_rd(&r, &info, text, CONSOLE_LOG_MAX); - *handover = false; + if (handover) + *handover = false; if (!prb_read_valid(prb, con->seq, &r)) return false; @@ -2703,7 +2891,8 @@ static bool console_emit_next_record(struct console *con, char *text, char *ext_ if (con->seq != r.info->seq) { con->dropped += r.info->seq - con->seq; con->seq = r.info->seq; - if (panic_in_progress() && panic_console_dropped++ > 10) { + if (panic_in_progress() && + atomic_fetch_inc_relaxed(&panic_console_dropped) > 10) { suppress_panic_printk = 1; pr_warn_once("Too many dropped messages. Suppress messages on non-panic CPUs to prevent livelock.\n"); } @@ -2725,31 +2914,61 @@ static bool console_emit_next_record(struct console *con, char *text, char *ext_ len = record_print_text(&r, console_msg_format & MSG_FORMAT_SYSLOG, printk_time); } - /* - * 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. - * - * Interrupts are disabled because the hand over to a waiter - * must not be interrupted until the hand over is completed - * (@console_waiter is cleared). - */ - printk_safe_enter_irqsave(flags); - console_lock_spinning_enable(); + if (handover) { + /* + * 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. + * + * Interrupts are disabled because the hand over to a waiter + * must not be interrupted until the hand over is completed + * (@console_waiter is cleared). + */ + printk_safe_enter_irqsave(flags); + console_lock_spinning_enable(); + + /* don't trace irqsoff print latency */ + stop_critical_timings(); + } - stop_critical_timings(); /* don't trace print latency */ call_console_driver(con, write_text, len, dropped_text); - start_critical_timings(); con->seq++; - *handover = console_lock_spinning_disable_and_check(); - printk_safe_exit_irqrestore(flags); + if (handover) { + start_critical_timings(); + *handover = console_lock_spinning_disable_and_check(); + printk_safe_exit_irqrestore(flags); + } skip: return true; } +/* + * Print a record for a given console, but allow another printk() caller to + * take over the console_lock and continue printing. + * + * Requires the console_lock, but depending on @handover after the call, the + * caller may no longer have the console_lock. + * + * See __console_emit_next_record() for argument and return details. + */ +static bool console_emit_next_record_transferable(struct console *con, char *text, char *ext_text, + char *dropped_text, bool *handover) +{ + /* + * Handovers are only supported if threaded printers are atomically + * blocked. The context taking over the console_lock may be atomic. + */ + if (!console_kthreads_atomically_blocked()) { + *handover = false; + handover = NULL; + } + + return __console_emit_next_record(con, text, ext_text, dropped_text, handover); +} + /* * Print out all remaining records to all consoles. * @@ -2768,8 +2987,8 @@ static bool console_emit_next_record(struct console *con, char *text, char *ext_ * were flushed to all usable consoles. A returned false informs the caller * that everything was not flushed (either there were no usable consoles or * another context has taken over printing or it is a panic situation and this - * is not the panic CPU). Regardless the reason, the caller should assume it - * is not useful to immediately try again. + * is not the panic CPU or direct printing is not preferred). Regardless the + * reason, the caller should assume it is not useful to immediately try again. * * Requires the console_lock. */ @@ -2786,6 +3005,10 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove *handover = false; do { + /* Let the kthread printers do the work if they can. */ + if (!allow_direct_printing()) + return false; + any_progress = false; for_each_console(con) { @@ -2797,13 +3020,11 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove if (con->flags & CON_EXTENDED) { /* Extended consoles do not print "dropped messages". */ - progress = console_emit_next_record(con, &text[0], - &ext_text[0], NULL, - handover); + progress = console_emit_next_record_transferable(con, &text[0], + &ext_text[0], NULL, handover); } else { - progress = console_emit_next_record(con, &text[0], - NULL, &dropped_text[0], - handover); + progress = console_emit_next_record_transferable(con, &text[0], + NULL, &dropped_text[0], handover); } if (*handover) return false; @@ -2918,10 +3139,13 @@ void console_unblank(void) if (oops_in_progress) { if (down_trylock_console_sem() != 0) return; + if (!console_kthreads_atomic_tryblock()) { + up_console_sem(); + return; + } } else console_lock(); - console_locked = 1; console_may_schedule = 0; for_each_console(c) if ((c->flags & CON_ENABLED) && c->unblank) @@ -3197,6 +3421,10 @@ void register_console(struct console *newcon) } newcon->dropped = 0; + newcon->thread = NULL; + newcon->blocked = true; + mutex_init(&newcon->lock); + if (newcon->flags & CON_PRINTBUFFER) { /* Get a consistent copy of @syslog_seq. */ mutex_lock(&syslog_lock); @@ -3206,6 +3434,10 @@ void register_console(struct console *newcon) /* Begin with next message. */ newcon->seq = prb_next_seq(prb); } + + if (printk_kthreads_available) + printk_start_kthread(newcon); + console_unlock(); console_sysfs_notify(); @@ -3229,6 +3461,7 @@ EXPORT_SYMBOL(register_console); int unregister_console(struct console *console) { + struct task_struct *thd; struct console *con; int res; @@ -3266,7 +3499,20 @@ int unregister_console(struct console *console) console_drivers->flags |= CON_CONSDEV; console->flags &= ~CON_ENABLED; + + /* + * console->thread can only be cleared under the console lock. But + * stopping the thread must be done without the console lock. The + * task that clears @thread is the task that stops the kthread. + */ + thd = console->thread; + console->thread = NULL; + console_unlock(); + + if (thd) + kthread_stop(thd); + console_sysfs_notify(); if (console->exit) @@ -3362,6 +3608,20 @@ static int __init printk_late_init(void) } late_initcall(printk_late_init); +static int __init printk_activate_kthreads(void) +{ + struct console *con; + + console_lock(); + printk_kthreads_available = true; + for_each_console(con) + printk_start_kthread(con); + console_unlock(); + + return 0; +} +early_initcall(printk_activate_kthreads); + #if defined CONFIG_PRINTK /* If @con is specified, only wait for that console. Otherwise wait for all. */ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress) @@ -3444,11 +3704,208 @@ static bool pr_flush(int timeout_ms, bool reset_on_progress) return __pr_flush(NULL, timeout_ms, reset_on_progress); } +static void __printk_fallback_preferred_direct(void) +{ + printk_prefer_direct_enter(); + pr_err("falling back to preferred direct printing\n"); + printk_kthreads_available = false; +} + +/* + * Enter preferred direct printing, but never exit. Mark console threads as + * unavailable. The system is then forever in preferred direct printing and + * any printing threads will exit. + * + * Must *not* be called under console_lock. Use + * __printk_fallback_preferred_direct() if already holding console_lock. + */ +static void printk_fallback_preferred_direct(void) +{ + console_lock(); + __printk_fallback_preferred_direct(); + console_unlock(); +} + +/* + * Print a record for a given console, not allowing another printk() caller + * to take over. This is appropriate for contexts that do not have the + * console_lock. + * + * See __console_emit_next_record() for argument and return details. + */ +static bool console_emit_next_record(struct console *con, char *text, char *ext_text, + char *dropped_text) +{ + return __console_emit_next_record(con, text, ext_text, dropped_text, NULL); +} + +static bool printer_should_wake(struct console *con, u64 seq) +{ + short flags; + + if (kthread_should_stop() || !printk_kthreads_available) + return true; + + if (con->blocked || + console_kthreads_atomically_blocked() || + block_console_kthreads || + system_state > SYSTEM_RUNNING || + oops_in_progress) { + return false; + } + + /* + * This is an unsafe read from con->flags, but a false positive is + * not a problem. Worst case it would allow the printer to wake up + * although it is disabled. But the printer will notice that when + * attempting to print and instead go back to sleep. + */ + flags = data_race(READ_ONCE(con->flags)); + + if (!__console_is_usable(flags)) + return false; + + return prb_read_valid(prb, seq, NULL); +} + +static int printk_kthread_func(void *data) +{ + struct console *con = data; + char *dropped_text = NULL; + char *ext_text = NULL; + u64 seq = 0; + char *text; + int error; + + text = kmalloc(CONSOLE_LOG_MAX, GFP_KERNEL); + if (!text) { + con_printk(KERN_ERR, con, "failed to allocate text buffer\n"); + printk_fallback_preferred_direct(); + goto out; + } + + if (con->flags & CON_EXTENDED) { + ext_text = kmalloc(CONSOLE_EXT_LOG_MAX, GFP_KERNEL); + if (!ext_text) { + con_printk(KERN_ERR, con, "failed to allocate ext_text buffer\n"); + printk_fallback_preferred_direct(); + goto out; + } + } else { + dropped_text = kmalloc(DROPPED_TEXT_MAX, GFP_KERNEL); + if (!dropped_text) { + con_printk(KERN_ERR, con, "failed to allocate dropped_text buffer\n"); + printk_fallback_preferred_direct(); + goto out; + } + } + + con_printk(KERN_INFO, con, "printing thread started\n"); + for (;;) { + /* + * Guarantee this task is visible on the waitqueue before + * checking the wake condition. + * + * The full memory barrier within set_current_state() of + * prepare_to_wait_event() pairs with the full memory barrier + * within wq_has_sleeper(). + * + * This pairs with __wake_up_klogd:A. + */ + error = wait_event_interruptible(log_wait, + printer_should_wake(con, seq)); /* LMM(printk_kthread_func:A) */ + + if (kthread_should_stop() || !printk_kthreads_available) + break; + + if (error) + continue; + + error = mutex_lock_interruptible(&con->lock); + if (error) + continue; + + if (con->blocked || + !console_kthread_printing_tryenter()) { + /* Another context has locked the console_lock. */ + mutex_unlock(&con->lock); + continue; + } + + /* + * Although this context has not locked the console_lock, it + * is known that the console_lock is not locked and it is not + * possible for any other context to lock the console_lock. + * Therefore it is safe to read con->flags. + */ + + if (!__console_is_usable(con->flags)) { + console_kthread_printing_exit(); + mutex_unlock(&con->lock); + continue; + } + + /* + * Even though the printk kthread is always preemptible, it is + * still not allowed to call cond_resched() from within + * console drivers. The task may become non-preemptible in the + * console driver call chain. For example, vt_console_print() + * takes a spinlock and then can call into fbcon_redraw(), + * which can conditionally invoke cond_resched(). + */ + console_may_schedule = 0; + console_emit_next_record(con, text, ext_text, dropped_text); + + seq = con->seq; + + console_kthread_printing_exit(); + + mutex_unlock(&con->lock); + } + + con_printk(KERN_INFO, con, "printing thread stopped\n"); +out: + kfree(dropped_text); + kfree(ext_text); + kfree(text); + + console_lock(); + /* + * If this kthread is being stopped by another task, con->thread will + * already be NULL. That is fine. The important thing is that it is + * NULL after the kthread exits. + */ + con->thread = NULL; + console_unlock(); + + return 0; +} + +/* Must be called under console_lock. */ +static void printk_start_kthread(struct console *con) +{ + /* + * Do not start a kthread if there is no write() callback. The + * kthreads assume the write() callback exists. + */ + if (!con->write) + return; + + con->thread = kthread_run(printk_kthread_func, con, + "pr/%s%d", con->name, con->index); + if (IS_ERR(con->thread)) { + con->thread = NULL; + con_printk(KERN_ERR, con, "unable to start printing thread\n"); + __printk_fallback_preferred_direct(); + return; + } +} + /* * Delayed printk version, for scheduler-internal messages: */ -#define PRINTK_PENDING_WAKEUP 0x01 -#define PRINTK_PENDING_OUTPUT 0x02 +#define PRINTK_PENDING_WAKEUP 0x01 +#define PRINTK_PENDING_DIRECT_OUTPUT 0x02 static DEFINE_PER_CPU(int, printk_pending); @@ -3456,10 +3913,14 @@ 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 (pending & PRINTK_PENDING_DIRECT_OUTPUT) { + printk_prefer_direct_enter(); + /* If trylock fails, someone else is doing the printing */ if (console_trylock()) console_unlock(); + + printk_prefer_direct_exit(); } if (pending & PRINTK_PENDING_WAKEUP) @@ -3484,10 +3945,11 @@ static void __wake_up_klogd(int val) * prepare_to_wait_event(), which is called after ___wait_event() adds * the waiter but before it has checked the wait condition. * - * This pairs with devkmsg_read:A and syslog_print:A. + * This pairs with devkmsg_read:A, syslog_print:A, and + * printk_kthread_func:A. */ if (wq_has_sleeper(&log_wait) || /* LMM(__wake_up_klogd:A) */ - (val & PRINTK_PENDING_OUTPUT)) { + (val & PRINTK_PENDING_DIRECT_OUTPUT)) { this_cpu_or(printk_pending, val); irq_work_queue(this_cpu_ptr(&wake_up_klogd_work)); } @@ -3527,7 +3989,17 @@ void defer_console_output(void) * New messages may have been added directly to the ringbuffer * using vprintk_store(), so wake any waiters as well. */ - __wake_up_klogd(PRINTK_PENDING_WAKEUP | PRINTK_PENDING_OUTPUT); + int val = PRINTK_PENDING_WAKEUP; + + /* + * Make sure that some context will print the messages when direct + * printing is allowed. This happens in situations when the kthreads + * may not be as reliable or perhaps unusable. + */ + if (allow_direct_printing()) + val |= PRINTK_PENDING_DIRECT_OUTPUT; + + __wake_up_klogd(val); } void printk_trigger_flush(void) diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c index 6d10927a07d8..8e8fd2fb0a5b 100644 --- a/kernel/printk/printk_safe.c +++ b/kernel/printk/printk_safe.c @@ -8,7 +8,9 @@ #include #include #include +#include #include +#include #include "internal.h" @@ -45,3 +47,33 @@ asmlinkage int vprintk(const char *fmt, va_list args) return vprintk_default(fmt, args); } EXPORT_SYMBOL(vprintk); + +/** + * try_block_console_kthreads() - Try to block console kthreads and + * make the global console_lock() avaialble + * + * @timeout_ms: The maximum time (in ms) to wait. + * + * Prevent console kthreads from starting processing new messages. Wait + * until the global console_lock() become available. + * + * Context: Can be called in any context. + */ +void try_block_console_kthreads(int timeout_ms) +{ + block_console_kthreads = true; + + /* Do not wait when the console lock could not be safely taken. */ + if (this_cpu_read(printk_context) || in_nmi()) + return; + + while (timeout_ms > 0) { + if (console_trylock()) { + console_unlock(); + return; + } + + udelay(1000); + timeout_ms -= 1; + } +} diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h index 7d15b5b5a235..7b411d663e8f 100644 --- a/kernel/rcu/tree_stall.h +++ b/kernel/rcu/tree_stall.h @@ -648,6 +648,7 @@ static void print_cpu_stall(unsigned long gps) * See Documentation/RCU/stallwarn.rst for info on how to debug * RCU CPU stall warnings. */ + printk_prefer_direct_enter(); trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected")); pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name); raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags); @@ -682,6 +683,7 @@ static void print_cpu_stall(unsigned long gps) */ set_tsk_need_resched(current); set_preempt_need_resched(); + printk_prefer_direct_exit(); } static void check_cpu_stall(struct rcu_data *rdp) diff --git a/kernel/reboot.c b/kernel/reboot.c index 6ebef11c8876..23a8cfed1a72 100644 --- a/kernel/reboot.c +++ b/kernel/reboot.c @@ -83,6 +83,7 @@ void kernel_restart_prepare(char *cmd) { blocking_notifier_call_chain(&reboot_notifier_list, SYS_RESTART, cmd); system_state = SYSTEM_RESTART; + try_block_console_kthreads(10000); usermodehelper_disable(); device_shutdown(); } @@ -283,6 +284,7 @@ static void kernel_shutdown_prepare(enum system_states state) blocking_notifier_call_chain(&reboot_notifier_list, (state == SYSTEM_HALT) ? SYS_HALT : SYS_POWER_OFF, NULL); system_state = state; + try_block_console_kthreads(10000); usermodehelper_disable(); device_shutdown(); } @@ -837,9 +839,11 @@ static int __orderly_reboot(void) ret = run_cmd(reboot_cmd); if (ret) { + printk_prefer_direct_enter(); pr_warn("Failed to start orderly reboot: forcing the issue\n"); emergency_sync(); kernel_restart(NULL); + printk_prefer_direct_exit(); } return ret; @@ -852,6 +856,7 @@ static int __orderly_poweroff(bool force) ret = run_cmd(poweroff_cmd); if (ret && force) { + printk_prefer_direct_enter(); pr_warn("Failed to start orderly shutdown: forcing the issue\n"); /* @@ -861,6 +866,7 @@ static int __orderly_poweroff(bool force) */ emergency_sync(); kernel_power_off(); + printk_prefer_direct_exit(); } return ret; @@ -918,6 +924,8 @@ EXPORT_SYMBOL_GPL(orderly_reboot); */ static void hw_failure_emergency_poweroff_func(struct work_struct *work) { + printk_prefer_direct_enter(); + /* * We have reached here after the emergency shutdown waiting period has * expired. This means orderly_poweroff has not been able to shut off @@ -934,6 +942,8 @@ static void hw_failure_emergency_poweroff_func(struct work_struct *work) */ pr_emerg("Hardware protection shutdown failed. Trying emergency restart\n"); emergency_restart(); + + printk_prefer_direct_exit(); } static DECLARE_DELAYED_WORK(hw_failure_emergency_poweroff_work, @@ -972,11 +982,13 @@ void hw_protection_shutdown(const char *reason, int ms_until_forced) { static atomic_t allow_proceed = ATOMIC_INIT(1); + printk_prefer_direct_enter(); + pr_emerg("HARDWARE PROTECTION shutdown (%s)\n", reason); /* Shutdown should be initiated only once. */ if (!atomic_dec_and_test(&allow_proceed)) - return; + goto out; /* * Queue a backup emergency shutdown in the event of @@ -984,6 +996,8 @@ void hw_protection_shutdown(const char *reason, int ms_until_forced) */ hw_failure_emergency_poweroff(ms_until_forced); orderly_poweroff(true); +out: + printk_prefer_direct_exit(); } EXPORT_SYMBOL_GPL(hw_protection_shutdown); diff --git a/kernel/watchdog.c b/kernel/watchdog.c index 45693fb3e08d..f366008298ac 100644 --- a/kernel/watchdog.c +++ b/kernel/watchdog.c @@ -431,6 +431,8 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer) /* Start period for the next softlockup warning. */ update_report_ts(); + printk_prefer_direct_enter(); + pr_emerg("BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n", smp_processor_id(), duration, current->comm, task_pid_nr(current)); @@ -449,6 +451,8 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer) add_taint(TAINT_SOFTLOCKUP, LOCKDEP_STILL_OK); if (softlockup_panic) panic("softlockup: hung tasks"); + + printk_prefer_direct_exit(); } return HRTIMER_RESTART; diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c index 1e8a49dc956e..7c977b945c92 100644 --- a/kernel/watchdog_hld.c +++ b/kernel/watchdog_hld.c @@ -135,6 +135,8 @@ static void watchdog_overflow_callback(struct perf_event *event, if (__this_cpu_read(hard_watchdog_warn) == true) return; + printk_prefer_direct_enter(); + pr_emerg("Watchdog detected hard LOCKUP on cpu %d\n", this_cpu); print_modules(); @@ -155,6 +157,8 @@ static void watchdog_overflow_callback(struct perf_event *event, if (hardlockup_panic) nmi_panic(regs, "Hard LOCKUP"); + printk_prefer_direct_exit(); + __this_cpu_write(hard_watchdog_warn, true); return; } -- 2.43.0