perf: Fix check before add_event_to_groups() in perf_group_detach()

Events should only be added to a groups rb tree if they have not been
removed from their context by list_del_event(). Since remove_on_exec
made it possible to call list_del_event() on individual events before
they are detached from their group, perf_group_detach() should check each
sibling's attach_state before calling add_event_to_groups() on it.

Fixes: 2e498d0 ("perf: Add support for event removal on exec")
Signed-off-by: Budimir Markovic <markovicbudimir@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/ZBFzvQV9tEqoHEtH@gentoo

Author: markovicbudimir

kernel/events/core.c

@@ -2163,7 +2163,7 @@ static void perf_group_detach(struct perf_event *event)
 		/* Inherit group flags from the previous leader */
 		sibling->group_caps = event->group_caps;
 
-		if (!RB_EMPTY_NODE(&event->group_node)) {
+		if (sibling->attach_state & PERF_ATTACH_CONTEXT) {
 			add_event_to_groups(sibling, event->ctx);
 
 			if (sibling->state == PERF_EVENT_STATE_ACTIVE)