[PATCH 4.14 83/89] printk: Wake klogd when passing console_lock owner

Greg Kroah-Hartman
2018-12-14T12:29:03Z

[ Upstream commit c14376de3a1befa70d9811ca2872d47367b48767 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 2f654a79f80b..2e2c86dd226f 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2393,7 +2393,7 @@ void console_unlock(void)

if (console_lock_spinning_disable_and_check()) {
printk_safe_exit_irqrestore(flags);
- return;
+ goto out;
}

printk_safe_exit_irqrestore(flags);
@@ -2426,6 +2426,7 @@ void console_unlock(void)
if (retry && console_trylock())
goto again;

+out:
if (wake_klogd)
wake_up_klogd();
}
--
2.19.1