perf: Fix exit() vs PERF_FORMAT_GROUP

Both Stephane and Corey reported that PERF_FORMAT_GROUP didn't work
as expected if the task the counters were attached to quit before
the read() call.

The cause is that we unconditionally destroy the grouping when we
remove counters from their context. Fix this by only doing this when
we free the counter itself.
Reported-by: NCorey Ashford <cjashfor@linux.vnet.ibm.com>
Reported-by: NStephane Eranian <eranian@google.com>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1273160566.5605.404.camel@twins>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

include/linux/perf_event.h

@@ -522,6 +522,7 @@ struct pmu {
  * enum perf_event_active_state - the states of a event
  */
 enum perf_event_active_state {
+	PERF_EVENT_STATE_FREE		= -3,
 	PERF_EVENT_STATE_ERROR		= -2,
 	PERF_EVENT_STATE_OFF		= -1,
 	PERF_EVENT_STATE_INACTIVE	=  0,

kernel/perf_event.c

@@ -341,6 +341,9 @@ list_del_event(struct perf_event *event, struct perf_event_context *ctx)
 	if (event->state > PERF_EVENT_STATE_OFF)
 		event->state = PERF_EVENT_STATE_OFF;
 
+	if (event->state > PERF_EVENT_STATE_FREE)
+		return;
+
 	/*
 	 * If this was a group event with sibling events then
 	 * upgrade the siblings to singleton events by adding them
@@ -1856,6 +1859,8 @@ int perf_event_release_kernel(struct perf_event *event)
 {
 	struct perf_event_context *ctx = event->ctx;
 
+	event->state = PERF_EVENT_STATE_FREE;
+
 	WARN_ON_ONCE(ctx->parent_ctx);
 	mutex_lock(&ctx->mutex);
 	perf_event_remove_from_context(event);