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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: Thu, 27 Jan 2011 18:11:34 +0100
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On Thu, 2011-01-27 at 17:57 +0100, Oleg Nesterov wrote:
> 
> > With, however, things are more interesting. 2 seems to be adequately
> > covered by the patch I just send, the IPI will bail and the next
> > sched-in of the relevant task will pick matters up. 1 otoh doesn't seem
> > covered, the IPI will bail, leaving us stranded.
> 
> Hmm, yes... Strangely, I missed that when I was thinking about in_ctxsw.
> 
> Perhaps, we can change task_oncpu_function_call() so that it returns
> -EAGAIN in case it hits in_ctxsw != 0? If the caller sees -EAGAIN, it
> should always retry even if !ctx->is_active.

That would be very easy to do, we can pass a return value through the
task_function_call structure.