perf: Fix group moving

Matt found we trigger the WARN_ON_ONCE() in perf_group_attach() when we take
the move_group path in perf_event_open().

Since we cannot de-construct the group (we rely on it to move the events), we
have to simply ignore the double attach. The group state is context invariant
and doesn't need changing.

Reported-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1287135757.29097.1368.camel@twins>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 6 insertions, 1 deletions

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 99b9700e74d0..346dc0e35a0a 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -315,7 +315,12 @@ static void perf_group_attach(struct perf_event *event)
 {
         struct perf_event *group_leader = event->group_leader;
 
-        WARN_ON_ONCE(event->attach_state & PERF_ATTACH_GROUP);
+        /*
+         * We can have double attach due to group movement in perf_event_open.
+         */
+        if (event->attach_state & PERF_ATTACH_GROUP)
+                return;
+
         event->attach_state |= PERF_ATTACH_GROUP;
 
         if (group_leader == event)