linux-kernel - Re: [PATCH] perf_event: fix loss of notification with multi-event sampling

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Date:	Tue, 22 Nov 2011 15:28:02 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu
Subject: Re: [PATCH] perf_event: fix loss of notification with multi-event sampling

On Tue, Nov 22, 2011 at 3:13 PM, Stephane Eranian <eranian@...gle.com> wrote:
> On Tue, Nov 22, 2011 at 2:29 PM, Peter Zijlstra <peterz@...radead.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.
>>
> I looked at how this is done for regular files: eventpoll_release(file);

Well, but's that called as part of __fput(), which is called before
fops->release().
So it should do it, unless you have to set some more flags in poll(),
i.e., more than
POLLIN.
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