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

[Alexander Shishkin <alexander.shishkin@linux.intel.com>]

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.

Thanks,
--
Alex