Messages in this thread | | | Subject | Re: perf_events: question about __perf_event_read() | From | Peter Zijlstra <> | Date | Fri, 21 Jan 2011 19:18:45 +0100 |
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On Fri, 2011-01-21 at 19:06 +0100, Stephane Eranian wrote: > Hi, > > I think the code below still has a problem in case of a per-cpu event. > > If you issue a read() on a different CPU, then you IPI to the event's cpu. > By the time you get there, the event may be de-scheduled in which > case you don't want to issue event->pmu_read() nor update context > timings. The function has a test but it seems to be checking the per-cpu > case only. > > I have seen panics on P4 with this code because it goes all the way > down to rdmsrl() with a bogus counter index (like -1). > > Am I missing something here? > > static void __perf_event_read(void *info) > { > struct perf_event *event = info; > struct perf_event_context *ctx = event->ctx; > struct perf_cpu_context *cpuctx = __get_cpu_context(ctx); > > /* > * If this is a task context, we need to check whether it is > * the current task context of this cpu. If not it has been > * scheduled out before the smp call arrived. In that case > * event->count would have been updated to a recent sample > * when the event was scheduled out. > */ > if (ctx->task && cpuctx->task_ctx != ctx) > return; > > raw_spin_lock(&ctx->lock);
Shouldn't we re-check event->state once we hold this lock?
> update_context_time(ctx); > update_event_times(event); > raw_spin_unlock(&ctx->lock); > > event->pmu->read(event); > }
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