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SubjectRe: [PATCH v0 3/5] perf: Introduce instruction trace filtering
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Peter Zijlstra <peterz@infradead.org> writes:

> 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.

Ah, but pmu::itrace_filter_setup() does not touch the hardware,
pmu::start() does. The former keeps an array of, say, MSR values ready
for programming in event::hw and the latter actually writes the MSRs. So
the above example should not be a problem.

So in a way validation and programming are split already. And PT, for
example, won't have it any other way, you can only program stuff into
the registers while tracing is disabled.

Regards,
--
Alex


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