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Subject: Re: Q: perf_install_in_context/perf_event_enable are racy?
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>, Ingo Molnar <mingo@elte.hu>,
        Alan Stern <stern@rowland.harvard.edu>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
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        Roland McGrath <roland@redhat.com>, linux-kernel@vger.kernel.org
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Date: Wed, 26 Jan 2011 20:05:31 +0100
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On Wed, 2011-01-26 at 19:49 +0100, Oleg Nesterov wrote:
> On 01/26, Oleg Nesterov wrote:
> >
> > Please see the untested patch below. It doesn't change perf_event_enable(),
> > only perf_install_in_context().
> 
> Forgot to mention... Also, it doesn't try to fix the race with do_exit(),
> this needs another change.
> 
> And, damn, can't resist. This is mostly cosmetic issue, but I feel
> discomfort every time I look at task_oncpu_function_call(). It _looks_
> obviously wrong, even if the problem doesn't exist in practice. I'll
> send the pedantic fix to keep the maintainers busy ;)

I've been trying to sit down and work my way through it today, your last
suggestion very nearly seemed to make sense, but I kept getting
distracted.

FWIW I think perf_event_enable() has the very same issue...