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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: Ingo Molnar <mingo@elte.hu>
Cc: Oleg Nesterov <oleg@redhat.com>, Alan Stern <stern@rowland.harvard.edu>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Paul Mackerras <paulus@samba.org>, Prasad <prasad@linux.vnet.ibm.com>,
        Roland McGrath <roland@redhat.com>, linux-kernel@vger.kernel.org
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Date: Fri, 21 Jan 2011 14:39:45 +0100
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On Fri, 2011-01-21 at 14:03 +0100, Ingo Molnar wrote:
> > > But, it seems, there is another problem. Forget about the exiting,
> > > I can't understand why we can trust current in the code above.
> > > With __ARCH_WANT_INTERRUPTS_ON_CTXSW schedule() does:
> > > 
> > >     // sets cpuctx->task_ctx = NULL
> > >     perf_event_task_sched_out();
> > > 
> > >     // enables irqs
> > >     prepare_lock_switch();
> > > 
> > > 
> > >     // updates current_task
> > >     switch_to();
> > > 
> > > What if IPI comes in the window before switch_to() ?
> > > 
> > > (the same questions for __perf_event_enable).
> > 
> > Ingo, do you have any insights in that, I think you wrote all that
> > initially?
> 
> Not sure. Can an IPI come there - we have irqs disabled usually, dont we?

Ah, I think I see how that works:

  __perf_event_task_sched_out()
    perf_event_context_sched_out()
      if (do_switch)
        cpuctx->task_ctx = NULL;

vs

  __perf_install_in_context()
   if (cpu_ctx->task_ctx != ctx)