linux-kernel - Re: Q: perf_install_in_context/perf_event

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Message-ID: <1295617185.28776.273.camel@laptop>
Date:	Fri, 21 Jan 2011 14:39:45 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Q: perf_install_in_context/perf_event_enable are racy?

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)


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