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Date: Fri, 11 Dec 2015 16:00:11 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        vince@deater.net, eranian@google.com,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Mathieu Poirier <mathieu.poirier@linaro.org>
Subject: Re: [PATCH v0 3/5] perf: Introduce instruction trace filtering
Message-ID: <20151211150011.GT6356@twins.programming.kicks-ass.net>
References: <1449840998-29902-1-git-send-email-alexander.shishkin@linux.intel.com>
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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;
> +}