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printk: Fix console_sem vs logbuf_lock unlock race

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Fix up the fallout from commit 0b5e1c5255 ("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: Peter 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: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Peter Zijlstra 2011-06-22 11:20:09 +02:00 committed by Ingo Molnar
parent 0b5e1c5255
commit 4f2a8d3cf5
1 changed files with 18 additions and 2 deletions
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20
kernel/printk.c
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@ -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();
}