linux-kernel - Re: Q: perf_install_in_context/perf_event

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Date:	Fri, 21 Jan 2011 14:03:23 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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?


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Thu, 2011-01-20 at 20:30 +0100, Oleg Nesterov wrote:
> > On 01/19, Oleg Nesterov wrote:
> > >
> > > Also. I believe there are more problems in perf_install_event(), but
> > > I need to recheck.
> > 
> > Help! I can't believe it can be so trivially wrong, but otoh I can't
> > understand how this can be correct.
> > 
> > So, ignoring details and !task case, __perf_install_in_context() does:
> > 
> > 	if (cpuctx->task_ctx || ctx->task != current)
> > 		return;
> > 
> > 	cpuctx->task_ctx = ctx;
> > 	event_sched_in(event);
> > 
> > Stupid question, what if this task has already passed
> > perf_event_exit_task() and thus it doesn't have ->perf_event_ctxp[] ?
> > Given that perf_event_context_sched_out() does nothing if !ctx, who
> > will event_sched_out() this event?
> > 
> > OK, even if I am right this is trivial, we just need the additional
> > check.
> 
> Indeed (or do the cleanup from put_ctx(), but that's too complex a
> change I think).
> 
> > 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?

Thanks,

	Ingo
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