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linux-stable/kernel/printk
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Petr Mladek 16c9a316f2 printk: Wake klogd when passing console_lock owner 
[ Upstream commit c14376de3a ]

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 dbdda842fe ("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: dbdda842fe ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
 		2018-12-17 09:28:55 +01:00
..
Makefile printk: introduce per-cpu safe_print seq buffer 2017-02-08 11:07:11 +01:00
braille.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
braille.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
console_cmdline.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
internal.h printk/nmi: Prevent deadlock when accessing the main log buffer in NMI 2018-09-05 09:26:35 +02:00
printk.c printk: Wake klogd when passing console_lock owner 2018-12-17 09:28:55 +01:00
printk_safe.c printk/tracing: Do not trace printk_nmi_enter() 2018-09-09 19:55:57 +02:00