Messages in this thread | | | Date | Fri, 11 Dec 2015 16:20:37 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v0 3/5] perf: Introduce instruction trace filtering |
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On Fri, Dec 11, 2015 at 04:02:36PM +0100, Peter Zijlstra wrote: > 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.
One solution is to keep a filter generation counter and check that on pmu::add(), and if mis-matched recompute the hardware setup at that point.
> > + } > > + > > + /* 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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