Peter Zijlstra <[email protected]> writes: > 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.
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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
