Subject: [PATCH 03/48] printk: Avoid non-panic CPUs writing to ringbuffer
From: John Ogness <[email-masked]>
Date: Wed, 7 Feb 2024 14:47:01 +0106

Commit 13fb0f74d702 ("printk: Avoid livelock with heavy printk
during panic") introduced a mechanism to silence non-panic CPUs
if too many messages are being dropped. Aside from trying to
workaround the livelock bugs of legacy consoles, it was also
intended to avoid losing panic messages. However, if non-panic
CPUs are writing to the ringbuffer, then reacting to dropped
messages is too late.

Another motivation is that non-finalized messages already might
be skipped in panic(). In other words, random messages from
non-panic CPUs might already get lost. It is better to ignore
all to avoid confusion.

To avoid losing panic CPU messages, silence non-panic CPUs
immediately on panic.

Signed-off-by: John Ogness <[email-masked]>
Reviewed-by: Petr Mladek <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Petr Mladek <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 kernel/printk/printk.c |   26 ++++++--------------------
 1 file changed, 6 insertions(+), 20 deletions(-)

Index: linux-6.8.2-rt10/kernel/printk/printk.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/printk/printk.c
+++ linux-6.8.2-rt10/kernel/printk/printk.c
@@ -462,12 +462,6 @@ static int console_msg_format = MSG_FORM
 static DEFINE_MUTEX(syslog_lock);
 
 #ifdef CONFIG_PRINTK
-/*
- * During panic, heavy printk by other CPUs can delay the
- * panic and risk deadlock on console resources.
- */
-static int __read_mostly suppress_panic_printk;
-
 DECLARE_WAIT_QUEUE_HEAD(log_wait);
 /* All 3 protected by @syslog_lock. */
 /* the next printk record to read by syslog(READ) or /proc/kmsg */
@@ -2322,7 +2316,12 @@ asmlinkage int vprintk_emit(int facility
 	if (unlikely(suppress_printk))
 		return 0;
 
-	if (unlikely(suppress_panic_printk) && other_cpu_in_panic())
+	/*
+	 * The messages on the panic CPU are the most important. If
+	 * non-panic CPUs are generating any messages, they will be
+	 * silently dropped.
+	 */
+	if (other_cpu_in_panic())
 		return 0;
 
 	if (level == LOGLEVEL_SCHED) {
@@ -2807,8 +2806,6 @@ void console_prepend_dropped(struct prin
 bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
 			     bool is_extended, bool may_suppress)
 {
-	static int panic_console_dropped;
-
 	struct printk_buffers *pbufs = pmsg->pbufs;
 	const size_t scratchbuf_sz = sizeof(pbufs->scratchbuf);
 	const size_t outbuf_sz = sizeof(pbufs->outbuf);
@@ -2836,17 +2833,6 @@ bool printk_get_next_message(struct prin
 	pmsg->seq = r.info->seq;
 	pmsg->dropped = r.info->seq - seq;
 
-	/*
-	 * Check for dropped messages in panic here so that printk
-	 * suppression can occur as early as possible if necessary.
-	 */
-	if (pmsg->dropped &&
-	    panic_in_progress() &&
-	    panic_console_dropped++ > 10) {
-		suppress_panic_printk = 1;
-		pr_warn_once("Too many dropped messages. Suppress messages on non-panic CPUs to prevent livelock.\n");
-	}
-
 	/* Skip record that has level above the console loglevel. */
 	if (may_suppress && suppress_message_printing(r.info->level))
 		goto out;