From: Petr Mladek <pmladek@suse.com>
Subject: [PATCH] printk: Wake klogd when passing console_lock owner
Git-commit: c14376de3a1befa70d9811ca2872d47367b48767
Patch-mainline: v4.16-rc4
References: bsc#1084301

wake_klogd is a local variable in console_unlock(). The information
is lost when the console_lock owner using the busy wait added by
the commit dbdda842fe96f8932 ("printk: Add console owner and waiter
logic to load balance console writes"). The following race is
possible:

CPU0				CPU1
console_unlock()

  for (;;)
     /* calling console for last message */

				printk()
				  log_store()
				    log_next_seq++;

     /* see new message */
     if (seen_seq != log_next_seq) {
	wake_klogd = true;
	seen_seq = log_next_seq;
     }

     console_lock_spinning_enable();

				  if (console_trylock_spinning())
				     /* spinning */

     if (console_lock_spinning_disable_and_check()) {
	printk_safe_exit_irqrestore(flags);
	return;

				  console_unlock()
				    if (seen_seq != log_next_seq) {
				    /* already seen */
				    /* nothing to do */

Result: Nobody would wakeup klogd.

One solution would be to make a global variable from wake_klogd.
But then we would need to manipulate it under a lock or so.

This patch wakes klogd also when console_lock is passed to the
spinning waiter. It looks like the right way to go. Also userspace
should have a chance to see and store any "flood" of messages.

Note that the very late klogd wake up was a historic solution.
It made sense on single CPU systems or when sys_syslog() operations
were synchronized using the big kernel lock like in v2.1.113.
But it is questionable these days.

Fixes: dbdda842fe96f8932 ("printk: Add console owner and waiter logic to load balance console writes")
Link: http://lkml.kernel.org/r/20180226155734.dzwg3aovqnwtvkoy@pathway.suse.cz
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org
Cc: Tejun Heo <tj@kernel.org>
Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/printk.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index db4b9b8929eb..4d818642ac0e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2397,7 +2397,7 @@ void console_unlock(void)
 
 		if (console_lock_spinning_disable_and_check()) {
 			printk_safe_exit_irqrestore(flags);
-			return;
+			goto out;
 		}
 
 		printk_safe_exit_irqrestore(flags);
@@ -2430,6 +2430,7 @@ void console_unlock(void)
 	if (retry && console_trylock())
 		goto again;
 
+out:
 	if (wake_klogd)
 		wake_up_klogd();
 }
-- 
2.13.6