From: Ryo Takakura <takakura@valinux.co.jp>

commit 779dbc2e78d7 ("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: 779dbc2e78d7 ("printk: Avoid non-panic CPUs writing to ringbuffer")
Signed-off-by: Ryo Takakura <takakura@valinux.co.jp>
---
 include/linux/panic.h  | 1 +
 kernel/panic.c         | 5 +++++
 kernel/printk/printk.c | 2 +-
 3 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/panic.h b/include/linux/panic.h
index 6717b15e7..556b4e2ad 100644
--- a/include/linux/panic.h
+++ b/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;
diff --git a/kernel/panic.c b/kernel/panic.c
index f94923a63..d7ed93567 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -63,6 +63,8 @@ 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);
 
@@ -253,7 +255,10 @@ 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) {
+		/* 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;
 		console_flush_on_panic(CONSOLE_FLUSH_PENDING);
 	}
 
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index d0bff0b0a..fa2580dd3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2354,7 +2354,7 @@ 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) {
-- 
2.34.1

On Sat 2024-08-03 17:16:49, takakura@valinux.co.jp wrote:
> From: Ryo Takakura <takakura@valinux.co.jp>
> 
> commit 779dbc2e78d7 ("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: 779dbc2e78d7 ("printk: Avoid non-panic CPUs writing to ringbuffer")
> Signed-off-by: Ryo Takakura <takakura@valinux.co.jp>

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr