[PATCH 1/3] sched/core: Avoid requeueing a singleton balance callback

[Andrea Righi <arighi@nvidia.com>]

With proxy execution, __schedule() can call set_next_task_scx() with
SNT_REPICK and then SNT_PICK for the same donor. Both calls queue the
kick_sync balance callback while kick_sync_pending remains set.

When the callback is the only list entry, its next pointer is NULL, so
the existing duplicate check misses it. Queueing it again links it to
itself and causes the wait to run twice.

Also check whether the callback is already the head of the list to
reject the second insertion.

Fixes: e2859566ffbc ("sched_ext: Queue the kick_sync wait from set_next_task_scx()")
Reported-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/08cac02c604dd73ef96fc92eb29d692b@kernel.org
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/sched.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 9e7993256c817..628765910934a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2175,7 +2175,8 @@ queue_balance_callback(struct rq *rq,
 	 * balance_push() is active, see the comment with
 	 * balance_push_callback.
 	 */
-	if (unlikely(head->next || rq->balance_callback == &balance_push_callback))
+	if (unlikely(head->next || rq->balance_callback == head ||
+		     rq->balance_callback == &balance_push_callback))
 		return;
 
 	head->func = func;
-- 
2.56.0