Commit e708d7ad authored Aug 04, 2014 by Sebastian Andrzej Siewior Committed by Ingo Molnar Aug 13, 2014

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: 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>

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;
		}

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