perf events: Fix ring_buffer_wakeup() brown paperbag bug

Commit 10c6db11 ("perf: Fix loss of notification with multi-event")
seems to unconditionally dereference event->rb in the wakeup handler,
this is wrong, there might not be a buffer attached.
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20111213152651.GP20297@mudshark.cambridge.arm.com
[ minor edits ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/events/core.c

@@ -3558,9 +3558,13 @@ static void ring_buffer_wakeup(struct perf_event *event)
 
 	rcu_read_lock();
 	rb = rcu_dereference(event->rb);
-	list_for_each_entry_rcu(event, &rb->event_list, rb_entry) {
+	if (!rb)
+		goto unlock;
+
+	list_for_each_entry_rcu(event, &rb->event_list, rb_entry)
 		wake_up_all(&event->waitq);
-	}
+
+unlock:
 	rcu_read_unlock();
 }