Messages in this thread Patch in this message | | | From | Sebastian Andrzej Siewior <> | Subject | [PATCH] perf: do poll_wait() before checking condition in perf_poll() | Date | Mon, 4 Aug 2014 15:31:08 +0200 |
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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> --- I haven't Cc stable@vger.kernel.org since nobody cared about this since day one.
kernel/events/core.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c index 6b17ac1..4aa3375 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -3528,6 +3528,7 @@ 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. @@ -3537,9 +3538,6 @@ 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; } -- 2.0.1
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