1 files changed, 163 insertions, 0 deletions
diff --git a/debian/patches-rt/0027-printk-Coordinate-direct-printing-in-panic.patch b/debian/patches-rt/0027-printk-Coordinate-direct-printing-in-panic.patch
new file mode 100644
index 000000000..4893dd463
--- /dev/null
+++ b/debian/patches-rt/0027-printk-Coordinate-direct-printing-in-panic.patch
@@ -0,0 +1,163 @@
+From: John Ogness <john.ogness@linutronix.de>
+Date: Wed, 22 Nov 2023 11:56:58 +0000
+Subject: [PATCH 27/48] printk: Coordinate direct printing in panic
+Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.8/older/patches-6.8.2-rt11.tar.xz
+
+Perform printing by nbcon consoles on the panic CPU from the
+printk() caller context in order to get panic messages printed
+as soon as possible.
+
+If legacy and nbcon consoles are registered, the legacy consoles
+will no longer perform direct printing on the panic CPU until
+after the backtrace has been stored. This will give the safe
+nbcon consoles a chance to print the panic messages before
+allowing the unsafe legacy consoles to print.
+
+If no nbcon consoles are registered, there is no change in
+behavior (i.e. legacy consoles will always attempt to print
+from the printk() caller context).
+
+Signed-off-by: John Ogness <john.ogness@linutronix.de>
+Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
+---
+ include/linux/printk.h |    5 +++
+ kernel/panic.c         |    2 +
+ kernel/printk/printk.c |   62 ++++++++++++++++++++++++++++++++++++++++++-------
+ 3 files changed, 61 insertions(+), 8 deletions(-)
+
+--- a/include/linux/printk.h
++++ b/include/linux/printk.h
+@@ -195,6 +195,7 @@ void show_regs_print_info(const char *lo
+ extern asmlinkage void dump_stack_lvl(const char *log_lvl) __cold;
+ extern asmlinkage void dump_stack(void) __cold;
+ void printk_trigger_flush(void);
++void printk_legacy_allow_panic_sync(void);
+ extern void nbcon_driver_acquire(struct console *con);
+ extern void nbcon_driver_release(struct console *con);
+ void nbcon_atomic_flush_unsafe(void);
+@@ -278,6 +279,10 @@ static inline void printk_trigger_flush(
+ {
+ }
+ 
++static inline void printk_legacy_allow_panic_sync(void)
++{
++}
++
+ static inline void nbcon_driver_acquire(struct console *con)
+ {
+ }
+--- a/kernel/panic.c
++++ b/kernel/panic.c
+@@ -364,6 +364,8 @@ void panic(const char *fmt, ...)
+ 
+ 	panic_other_cpus_shutdown(_crash_kexec_post_notifiers);
+ 
++	printk_legacy_allow_panic_sync();
++
+ 	/*
+ 	 * Run any panic handlers, including those that might need to
+ 	 * add information to the kmsg dump output.
+--- a/kernel/printk/printk.c
++++ b/kernel/printk/printk.c
+@@ -471,7 +471,9 @@ static DEFINE_MUTEX(syslog_lock);
+ static bool have_legacy_console;
+ 
+ /*
+- * Specifies if an nbcon console is registered.
++ * Specifies if an nbcon console is registered. If nbcon consoles are present,
++ * synchronous printing of legacy consoles will not occur during panic until
++ * the backtrace has been stored to the ringbuffer.
+  */
+ static bool have_nbcon_console;
+ 
+@@ -2335,12 +2337,29 @@ int vprintk_store(int facility, int leve
+ 	return ret;
+ }
+ 
++static bool legacy_allow_panic_sync;
++
++/*
++ * This acts as a one-way switch to allow legacy consoles to print from
++ * the printk() caller context on a panic CPU. It also attempts to flush
++ * the legacy consoles in this context.
++ */
++void printk_legacy_allow_panic_sync(void)
++{
++	legacy_allow_panic_sync = true;
++
++	if (printing_via_unlock && !in_nmi()) {
++		if (console_trylock())
++			console_unlock();
++	}
++}
++
+ asmlinkage int vprintk_emit(int facility, int level,
+ 			    const struct dev_printk_info *dev_info,
+ 			    const char *fmt, va_list args)
+ {
++	bool do_trylock_unlock = printing_via_unlock;
+ 	int printed_len;
+-	bool in_sched = false;
+ 
+ 	/* Suppress unimportant messages after panic happens */
+ 	if (unlikely(suppress_printk))
+@@ -2356,15 +2375,42 @@ asmlinkage int vprintk_emit(int facility
+ 
+ 	if (level == LOGLEVEL_SCHED) {
+ 		level = LOGLEVEL_DEFAULT;
+-		in_sched = true;
++		/* If called from the scheduler, we can not call up(). */
++		do_trylock_unlock = false;
+ 	}
+ 
+ 	printk_delay(level);
+ 
+ 	printed_len = vprintk_store(facility, level, dev_info, fmt, args);
+ 
+-	/* If called from the scheduler, we can not call up(). */
+-	if (!in_sched && printing_via_unlock) {
++	if (have_nbcon_console && !have_boot_console) {
++		bool is_panic_context = this_cpu_in_panic();
++
++		/*
++		 * In panic, the legacy consoles are not allowed to print from
++		 * the printk calling context unless explicitly allowed. This
++		 * gives the safe nbcon consoles a chance to print out all the
++		 * panic messages first. This restriction only applies if
++		 * there are nbcon consoles registered.
++		 */
++		if (is_panic_context)
++			do_trylock_unlock &= legacy_allow_panic_sync;
++
++		/*
++		 * There are situations where nbcon atomic printing should
++		 * happen in the printk() caller context:
++		 *
++		 * - When this CPU is in panic.
++		 *
++		 * Note that if boot consoles are registered, the console
++		 * lock/unlock dance must be relied upon instead because nbcon
++		 * consoles cannot print simultaneously with boot consoles.
++		 */
++		if (is_panic_context)
++			nbcon_atomic_flush_pending();
++	}
++
++	if (do_trylock_unlock) {
+ 		/*
+ 		 * The caller may be holding system-critical or
+ 		 * timing-sensitive locks. Disable preemption during
+@@ -2384,10 +2430,10 @@ asmlinkage int vprintk_emit(int facility
+ 		preempt_enable();
+ 	}
+ 
+-	if (in_sched)
+-		defer_console_output();
+-	else
++	if (do_trylock_unlock)
+ 		wake_up_klogd();
++	else
++		defer_console_output();
+ 
+ 	return printed_len;
+ }