On Fri, Dec 11, 2015 at 05:27:22PM +0200, Alexander Shishkin wrote:
> Peter Zijlstra <[email protected]> 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.
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