perf: Fix reading in perf_event_read()

It is quite possible for the event to have been disabled between
perf_event_read() sending the IPI and the CPU servicing the IPI and
calling __perf_event_read(), hence revalidate the state.

Reported-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 852ae8c..999835b 100644 (file)
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -1901,11 +1901,12 @@ static void __perf_event_read(void *info)
                return;
 
        raw_spin_lock(&ctx->lock);
-       update_context_time(ctx);
+       if (ctx->is_active)
+               update_context_time(ctx);
        update_event_times(event);
+       if (event->state == PERF_EVENT_STATE_ACTIVE)
+               event->pmu->read(event);
        raw_spin_unlock(&ctx->lock);
-
-       event->pmu->read(event);
 }
 
 static inline u64 perf_event_count(struct perf_event *event)