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Date: Fri, 11 Dec 2015 16:33:54 +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: <20151211153354.GY6356@twins.programming.kicks-ass.net>
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On Fri, Dec 11, 2015 at 05:27:22PM +0200, Alexander Shishkin wrote:
> 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?

Dunno, I worry :-)

What if:

	if (READ_ONCE(event->state) != PERF_EVENT_STATE_ACTIVE) {
		// we were INACTIVE, but now the event gets scheduled in
		// on _another_ CPU
		event->pmu->itrace_filter_setup() := {
			if (event->state == PERF_EVENT_STATE_ACTIVE) {
				/* muck with hardware */
			}
		}
	}

Here too I feel a strict validation vs programming split would make sense.

We can always call the validation thing, we must not call the program
thing !ACTIVE is a clear and simple rule.