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SubjectRe: perf_events: question about __perf_event_read()
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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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