printk/panic: Add option to allow non-panic CPUs to write to the ring buffer.

Commit 779dbc2e78d7 ("printk: Avoid non-panic CPUs writing to ringbuffer")
aimed to isolate panic-related messages. However, when panic() itself
malfunctions, messages from non-panic CPUs become crucial for debugging.

While commit bcc954c6caba ("printk/panic: Allow cpu backtraces to
be written into ringbuffer during panic") enables non-panic CPU
backtraces, it may not provide sufficient diagnostic information.

Introduce the "debug_non_panic_cpus" command-line option, enabling
non-panic CPU messages to be stored in the ring buffer during a panic.
This also prevents discarding non-finalized messages from non-panic CPUs
during console flushing, providing a more comprehensive view of system
state during critical failures.

Link: https://lore.kernel.org/all/Z8cLEkqLL2IOyNIj@pathway/
Signed-off-by: Donghyeok Choe <d7271.choe@samsung.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20250318022320.2428155-1-d7271.choe@samsung.com
[pmladek@suse.com: Added documentation, added module_parameter, removed printk_ prefix.]
Tested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

1 files changed, 19 insertions, 1 deletions

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 2590498d8353..74b4734a87b9 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2375,6 +2375,22 @@ void printk_legacy_allow_panic_sync(void)
 	}
 }
 
+bool __read_mostly debug_non_panic_cpus;
+
+#ifdef CONFIG_PRINTK_CALLER
+static int __init debug_non_panic_cpus_setup(char *str)
+{
+	debug_non_panic_cpus = true;
+	pr_info("allow messages from non-panic CPUs in panic()\n");
+
+	return 0;
+}
+early_param("debug_non_panic_cpus", debug_non_panic_cpus_setup);
+module_param(debug_non_panic_cpus, bool, 0644);
+MODULE_PARM_DESC(debug_non_panic_cpus,
+		 "allow messages from non-panic CPUs in panic()");
+#endif
+
 asmlinkage int vprintk_emit(int facility, int level,
 			    const struct dev_printk_info *dev_info,
 			    const char *fmt, va_list args)
@@ -2391,7 +2407,9 @@ 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() && !panic_triggering_all_cpu_backtrace)
+	if (other_cpu_in_panic() &&
+	    !debug_non_panic_cpus &&
+	    !panic_triggering_all_cpu_backtrace)
 		return 0;
 
 	printk_get_console_flush_type(&ft);