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From: Alexander Shishkin <alexander.shishkin@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
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
In-Reply-To: <20151211150236.GU6356@twins.programming.kicks-ass.net>
References: <1449840998-29902-1-git-send-email-alexander.shishkin@linux.intel.com> <1449840998-29902-4-git-send-email-alexander.shishkin@linux.intel.com> <20151211150236.GU6356@twins.programming.kicks-ass.net>
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Date: Fri, 11 Dec 2015 17:27:22 +0200
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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(struct perf_event *event)
>> +{
>> +	int ret;
>> +
>> +	/*
>> +	 * We can't use event_function_call() here, because that would
>> +	 * require ctx::mutex, but one of our callers is called with
>> +	 * mm::mmap_sem down, which would cause an inversion, see bullet
>> +	 * (2) in put_event().
>> +	 */
>> +	do {
>> +		if (READ_ONCE(event->state) != PERF_EVENT_STATE_ACTIVE) {
>> +			ret = event->pmu->itrace_filter_setup(event);
>> +			break;
>
> So this is tricky, if its not active it can be any moment, there is
> nothing serializing against that.

Indeed. But we should be able to call pmu::itrace_filter_setup()
multiple times, so if after this we re-check that the event is still
inactive, we can return, otherwise proceed with the cross-call. Does
this make sense?

>
>> +		}
>> +
>> +		/* matches smp_wmb() in event_sched_in() */
>> +		smp_rmb();
>> +
>> +		ret = cpu_function_call(READ_ONCE(event->oncpu),
>> +					__perf_event_itrace_filters_setup, event);
>
> This otoh, running with IRQs disabled on the CPU the thing is active on
> guarantees it will not become inactive -- nothing can come in and switch
> it off.
>
>> +	} while (ret == -EAGAIN);
>> +
>> +	return ret;
>> +}