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Subject: Re: [PATCH,RFC] perf: panic due to inclied cpu context task_ctx
 value
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>, Paul Mackerras <paulus@samba.org>,
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Date: Mon, 28 Mar 2011 17:49:28 +0200
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On Mon, 2011-03-28 at 16:57 +0200, Peter Zijlstra wrote:
> > 
> > Wait... Yes, we have to clear ctx->is_active, otherwise we break, say,
> > perf_install_in_context().
> > 
> > But if we clear ->is_active we break perf_event_enable(). Suppose we
> > are doing ioctl(PERF_EVENT_IOC_DISABLE) + ioctl(PERF_EVENT_IOC_ENABLE).
> > PERF_EVENT_IOC_DISABLE can sched_out the last event, but _IOC_ENABLE
> > treats ctx->is_active == F as "it is not running".
> 
> Right, same for the tick, if say we can only schedule 1 event at a time
> and we close the 1 event that is active, the tick will not rotate a new
> event in. 

Another fun race, suppose we do properly remove task_ctx and is_active,
but then the task gets scheduled back in before free_event() gets around
to disabling the jump_label..