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perf: Do poll_wait() before checking condition in perf_poll()

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One should first enqueue to the waitqueue and then check for the
condition. If the condition gets true after mutex_unlock() but before
poll_wait() then we lose it and would have wait for another wakeup.

This has been like this since v2.6.31-rc1 commit c7138f37f9 ("perf_counter:
fix perf_poll()"). Before that it was slightly worse. I guess we get enough
wakeups so if we miss here one it doesn't really matter. It is still a
bad example.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1407159068-1478-1-git-send-email-bigeasy@linutronix.de
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Sebastian Andrzej Siewior 2014-08-04 15:31:08 +02:00 committed by Ingo Molnar
parent 36bbb2f298
commit e708d7ad80
1 changed files with 1 additions and 3 deletions
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4
kernel/events/core.c
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@ -3629,6 +3629,7 @@ static unsigned int perf_poll(struct file *file, poll_table *wait)
struct ring_buffer *rb;
unsigned int events = POLL_HUP;
poll_wait(file, &event->waitq, wait);
/*
* Pin the event->rb by taking event->mmap_mutex; otherwise
* perf_event_set_output() can swizzle our rb and make us miss wakeups.
@ -3638,9 +3639,6 @@ static unsigned int perf_poll(struct file *file, poll_table *wait)
if (rb)
events = atomic_xchg(&rb->poll, 0);
mutex_unlock(&event->mmap_mutex);
poll_wait(file, &event->waitq, wait);
return events;
}