printk/panic: Allow cpu backtraces to be written into ringbuffer during panic

commit 779dbc2e ("printk: Avoid non-panic CPUs writing
to ringbuffer") disabled non-panic CPUs to further write messages to
ringbuffer after panicked.

Since the commit, non-panicked CPU's are not allowed to write to
ring buffer after panicked and CPU backtrace which is triggered
after panicked to sample non-panicked CPUs' backtrace no longer
serves its function as it has nothing to print.

Fix the issue by allowing non-panicked CPUs to write into ringbuffer
while CPU backtrace is in flight.

Fixes: 779dbc2e ("printk: Avoid non-panic CPUs writing to ringbuffer")
Signed-off-by: Ryo Takakura <takakura@valinux.co.jp>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240812072703.339690-1-takakura@valinux.co.jpSigned-off-by: Petr Mladek <pmladek@suse.com>

include/linux/panic.h

@@ -16,6 +16,7 @@ extern void oops_enter(void);
 extern void oops_exit(void);
 extern bool oops_may_print(void);
 
+extern bool panic_triggering_all_cpu_backtrace;
 extern int panic_timeout;
 extern unsigned long panic_print;
 extern int panic_on_oops;

kernel/panic.c

@@ -64,6 +64,8 @@ unsigned long panic_on_taint;
 bool panic_on_taint_nousertaint = false;
 static unsigned int warn_limit __read_mostly;
 
+bool panic_triggering_all_cpu_backtrace;
+
 int panic_timeout = CONFIG_PANIC_TIMEOUT;
 EXPORT_SYMBOL_GPL(panic_timeout);
 
@@ -253,8 +255,12 @@ void check_panic_on_warn(const char *origin)
  */
 static void panic_other_cpus_shutdown(bool crash_kexec)
 {
-	if (panic_print & PANIC_PRINT_ALL_CPU_BT)
+	if (panic_print & PANIC_PRINT_ALL_CPU_BT) {
+		/* Temporary allow non-panic CPUs to write their backtraces. */
+		panic_triggering_all_cpu_backtrace = true;
 		trigger_all_cpu_backtrace();
+		panic_triggering_all_cpu_backtrace = false;
+	}
 
 	/*
 	 * Note that smp_send_stop() is the usual SMP shutdown function,

kernel/printk/printk.c

@@ -2316,7 +2316,7 @@ asmlinkage int vprintk_emit(int facility, int level,
 	 * non-panic CPUs are generating any messages, they will be
 	 * silently dropped.
 	 */
-	if (other_cpu_in_panic())
+	if (other_cpu_in_panic() && !panic_triggering_all_cpu_backtrace)
 		return 0;
 
 	if (level == LOGLEVEL_SCHED) {