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

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Date:	Tue, 22 Nov 2011 14:29:19 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu
Subject: Re: [PATCH] perf_event: fix loss of notification with multi-event
 sampling

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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