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    SubjectRe: [PATCH] perf_event: fix loss of notification with multi-event sampling
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    On Tue, 2011-11-22 at 14:15 +0100, Stephane Eranian wrote:

    > > Ah, could it be a race of poll()/wakeup() vs perf_event_set_output() ?
    > >
    > Are you saying that by dropping event->waitq in favor of event->rb->waitq
    > we make this problem disappear due to rcu protections?

    Well, except..

    > Poll_wait() is a blocking call. It may wait on a stale waitq. But that problem
    > was probably already there. I am not clear as to what to do about that.
    > in perf_set_output() you would need to wakeup from poll_wait() and then
    > go back in with the new waitq.

    Right, the whole blocking thing is a problem, and the whole poll()
    interface always makes my head hurt.

    If there was a go-sleep and wake-up side to poll we could do
    ring_buffer_get()/put() and fix this problem, but I'm not finding a way
    to make that happen quite yet.

    > Similarly, I am not clear as to what happens when you close an event for
    > which you have a waiter in poll_wait(). I assume you wakeup from it.
    > But I don't see where that's implemented.

    Good point, yes we should do that.


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