Messages in this thread | | | Date | Wed, 23 Nov 2011 10:30:20 +0100 | Subject | Re: [PATCH] perf_event: fix loss of notification with multi-event sampling | From | Stephane Eranian <> |
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On Tue, Nov 22, 2011 at 2:29 PM, Peter Zijlstra <peterz@infradead.org> wrote: > 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. > So I did run some checks on the behavior of poll() in case a polled fd is closed. It appears that you don't get out of it in all cases. It depends on the nature of the file. I believe you get out of poll() ONLY in case of a producer-consumer file descriptor pair, e.g., a pipe, named pipe, socket. In other words you need to have a producer file descriptor. In case the producer is closed, the reader waiting in poll() returns with a POLLHUP flag. I did verify that for named pipes: close the writer, and the reader comes out of poll.
In the case of perf_event, we are not in a producer-consumer model, so it seems like the behavior we have now is correct. The caller of poll() gets stuck if the file descriptor is closed. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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