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Commit e708d7ad authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Ingo Molnar
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perf: Do poll_wait() before checking condition in perf_poll() 



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 c7138f37 ("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: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarPeter 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: default avatarIngo Molnar <mingo@kernel.org>
parent 36bbb2f2

kernel/events/core.c

+1 −3
Original line number Diff line number Diff line
@@ -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;

}