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Subjectperf_events: question about __perf_event_read()
From
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);
update_context_time(ctx);
update_event_times(event);
raw_spin_unlock(&ctx->lock);

event->pmu->read(event);
}


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