[PATCH] printk/panic: Add option allow non panic cpus logging to ringbuffer

[Donghyeok Choe <d7271.choe@samsung.com>]

commit 779dbc2e78d7 ("printk: Avoid non-panic CPUs writing
to ringbuffer") disabled non-panic CPUs to further write messages to
ringbuffer after panicked.

commit bcc954c6caba ("printk/panic: Allow cpu backtraces to
be written into ringbuffer during panic") allows non-panicked CPUs
to write backtrace only.

Since that, there was a problem with not being able to check the
important logs of the non-panicked CPUs.

Fix the issue by adding debug option(printk_debug_non_panic_cpus) to
output the non-paniced CPUs' message.

Link: https://lore.kernel.org/all/Z8cLEkqLL2IOyNIj@pathway/
Signed-off-by: Donghyeok Choe <d7271.choe@samsung.com>
---
 kernel/printk/internal.h          |  1 +
 kernel/printk/printk.c            | 17 ++++++++++++++++-
 kernel/printk/printk_ringbuffer.c |  4 +++-
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index a91bdf802967..22d193907bb8 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -64,6 +64,7 @@ struct dev_printk_info;
 
 extern struct printk_ringbuffer *prb;
 extern bool printk_kthreads_running;
+extern bool printk_debug_non_panic_cpus;
 
 __printf(4, 0)
 int vprintk_store(int facility, int level,
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 07668433644b..7631fd8ba1c7 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2375,6 +2375,19 @@ void printk_legacy_allow_panic_sync(void)
 	}
 }
 
+bool __read_mostly printk_debug_non_panic_cpus;
+
+#ifdef CONFIG_PRINTK_CALLER
+static int __init printk_debug_non_panic_cpus_setup(char *str)
+{
+	printk_debug_non_panic_cpus = true;
+	pr_info("printk: keep printk all cpu in panic.\n");
+
+	return 0;
+}
+early_param("printk_debug_non_panic_cpus", printk_debug_non_panic_cpus_setup);
+#endif
+
 asmlinkage int vprintk_emit(int facility, int level,
 			    const struct dev_printk_info *dev_info,
 			    const char *fmt, va_list args)
@@ -2391,7 +2404,9 @@ 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 (!printk_debug_non_panic_cpus &&
+	    !panic_triggering_all_cpu_backtrace &&
+	    other_cpu_in_panic())
 		return 0;
 
 	printk_get_console_flush_type(&ft);
diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 88e8f3a61922..ffab5f6c1855 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -2143,7 +2143,9 @@ static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
 			 * But it would have the sequence number returned
 			 * by "prb_next_reserve_seq() - 1".
 			 */
-			if (this_cpu_in_panic() && ((*seq + 1) < prb_next_reserve_seq(rb)))
+			if (this_cpu_in_panic() &&
+				(!printk_debug_non_panic_cpus || legacy_allow_panic_sync) &&
+				((*seq + 1) < prb_next_reserve_seq(rb)))
 				(*seq)++;
 			else
 				return false;
-- 
2.48.0