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

[Peter Zijlstra]

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?

> +
> +	return ret;
> +}
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