[PATCH v3 2/7] perf: Simplify child event tear-down

[Peter Zijlstra <peterz@infradead.org>]

Currently perf_event_release_kernel() will iterate the child events and attempt
tear-down. However, it removes them from the child_list using list_move(),
notably skipping the state management done by perf_child_detach().

Crucially, it fails to clear PERF_ATTACH_CHILD, which opens the door for a
concurrent perf_remove_from_context() to race.

This way child_list management stays fully serialized using child_mutex.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/events/core.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2338,7 +2338,11 @@ static void perf_child_detach(struct per
 	if (WARN_ON_ONCE(!parent_event))
 		return;
 
+	/*
+	 * Can't check this from an IPI, the holder is likey another CPU.
+	 *
 	lockdep_assert_held(&parent_event->child_mutex);
+	 */
 
 	sync_child_event(event);
 	list_del_init(&event->child_list);
@@ -5611,8 +5615,8 @@ int perf_event_release_kernel(struct per
 		tmp = list_first_entry_or_null(&event->child_list,
 					       struct perf_event, child_list);
 		if (tmp == child) {
-			perf_remove_from_context(child, DETACH_GROUP);
-			list_move(&child->child_list, &free_list);
+			perf_remove_from_context(child, DETACH_GROUP | DETACH_CHILD);
+			list_add(&child->child_list, &free_list);
 			/*
 			 * This matches the refcount bump in inherit_event();
 			 * this can't be the last reference.