Re: [PATCH v0 3/5] perf: Introduce instruction trace filtering

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Dec 11, 2015 at 05:17:25PM +0200, Alexander Shishkin wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
> 
> > On Fri, Dec 11, 2015 at 03:36:36PM +0200, Alexander Shishkin wrote:
> >> +static int __perf_event_itrace_filters_setup(void *info)
> >> +{
> >> +	struct perf_event *event = info;
> >> +	int ret;
> >> +
> >> +	if (READ_ONCE(event->state) != PERF_EVENT_STATE_ACTIVE)
> >> +		return -EAGAIN;
> >> +
> >> +	/* matches smp_wmb() in event_sched_in() */
> >> +	smp_rmb();
> >> +
> >> +	/*
> >> +	 * There is a window with interrupts enabled before we get here,
> >> +	 * so we need to check again lest we try to stop another cpu's event.
> >> +	 */
> >> +	if (READ_ONCE(event->oncpu) != smp_processor_id())
> >> +		return -EAGAIN;
> >> +
> >> +	event->pmu->stop(event, PERF_EF_UPDATE);
> >> +	rcu_read_lock();
> >> +	ret = event->pmu->itrace_filter_setup(event);
> >> +	rcu_read_unlock();
> >> +	event->pmu->start(event, PERF_EF_RELOAD);
> >
> > Would it not be more sensible to let the ::itrace_filter_setup() method
> > do the stop/start-ing if and when needed?
> 
> I don't have a strong opinion on this, the only question is, are we
> comfortable with pmu driver callback doing the
> rcu_read_lock/unlock, because it still needs to iterate the filter list.
> Other than that it's probably a good idea.

See another email; I'm not sure RCU works for this. You can observe more
events than you set out for, and if you do multiple iterations of the
list they need not match.