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Commit 4f2a8d3c authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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printk: Fix console_sem vs logbuf_lock unlock race 



Fix up the fallout from commit 0b5e1c52 ("printk: Release
console_sem after logbuf_lock").

The reason for unlocking the console_sem under the logbuf_lock
is that a concurrent printk() might fill up the buffer but fail
to acquire the console sem, resulting in a missed write to the
console until a subsequent console_sem acquire/release cycle.

Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: efault@gmx.de
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1308734409.1022.14.camel@twins


Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 0b5e1c52

kernel/printk.c

+18 −2
Original line number Diff line number Diff line
@@ -1244,7 +1244,7 @@ void console_unlock(void) 
{

	unsigned long flags;

	unsigned _con_start, _log_end;

	unsigned wake_klogd = 0;

	unsigned wake_klogd = 0, retry = 0;



	if (console_suspended) {

		up(&console_sem);
@@ -1253,6 +1253,7 @@ void console_unlock(void) 


	console_may_schedule = 0;



again:

	for ( ; ; ) {

		spin_lock_irqsave(&logbuf_lock, flags);

		wake_klogd |= log_start - log_end;
@@ -1273,8 +1274,23 @@ void console_unlock(void) 
	if (unlikely(exclusive_console))

		exclusive_console = NULL;



	spin_unlock_irqrestore(&logbuf_lock, flags);

	spin_unlock(&logbuf_lock);



	up(&console_sem);



	/*

	 * Someone could have filled up the buffer again, so re-check if there's

	 * something to flush. In case we cannot trylock the console_sem again,

	 * there's a new owner and the console_unlock() from them will do the

	 * flush, no worries.

	 */

	spin_lock(&logbuf_lock);

	if (con_start != log_end)

		retry = 1;

	spin_unlock_irqrestore(&logbuf_lock, flags);

	if (retry && console_trylock())

		goto again;



	if (wake_klogd)

		wake_up_klogd();

}