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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: Alan Stern <stern@rowland.harvard.edu>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, Ingo Molnar <mingo@elte.hu>,
        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 13:11:45 +0100
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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?