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Date: Mon, 25 Apr 2016 15:10:13 +0200
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 v1 4/5] perf: Introduce address range filtering
Message-ID: <20160425131013.GD3448@twins.programming.kicks-ass.net>
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On Fri, Apr 22, 2016 at 07:19:11PM +0300, Alexander Shishkin wrote:
> +static int perf_event_restart(struct perf_event *event)
> +{
> +	struct stop_event_data sd = {
> +		.event		= event,
> +		.restart	= 1,
> +	};
> +	int ret = 0;
> +
> +	do {
> +		if (READ_ONCE(event->state) != PERF_EVENT_STATE_ACTIVE)
> +			return 0;
> +
> +		/* matches smp_wmb() in event_sched_in() */
> +		smp_rmb();
> +
> +		ret = cpu_function_call(READ_ONCE(event->oncpu),
> +					__perf_event_stop, &sd);
> +	} while (ret == -EAGAIN);
> +
> +	return ret;
> +}

A few concerns with this;

 - if I understand things right; this will loose a bit of trace when
   people tickle the ioctl(), right? This might want a note of sorts,

 - you need to handle event->oncpu == -1,

 - can we race with an actual stop, and accidentally re-enable the
   thing? If not, this might be a good place for a comment explaining
   how.