[tip: perf/core] perf: Simplify child event tear-down

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the perf/core branch of tip:

Commit-ID:     0a00a43b8c200df5b9ca2b3e1726479b5916264b
Gitweb:        https://git.kernel.org/tip/0a00a43b8c200df5b9ca2b3e1726479b5916264b
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Fri, 17 Jan 2025 15:25:23 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 08 Apr 2025 20:55:46 +02:00

perf: Simplify child event tear-down

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@xxxxxxxxxxxxx>
Reviewed-by: Ravi Bangoria <ravi.bangoria@xxxxxxx>
Link: https://lkml.kernel.org/r/20250307193305.486326750@xxxxxxxxxxxxx
---
 kernel/events/core.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index a85d63b..3c92b75 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2325,7 +2325,11 @@ static void perf_child_detach(struct perf_event *event)
 	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);
@@ -5759,8 +5763,8 @@ again:
 		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);
 		} else {
 			var = &ctx->refcount;
 		}