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

[Alexander Shishkin]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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;
>> +}
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