[PATCH] sched: Recheck the rt task's on rq state after double_lock_balance()

[Tengfei Fan]

Recheck whether next_task is still in the runqueue of this_rq after
locking this_rq and lowest_rq via double_lock_balance() in
push_rt_task(). This is necessary because double_lock_balance() first
releases this_rq->lock and then attempts to acquire both this_rq->lock
and lowest_rq->lock, during which next_task may have already been
removed from this_rq's runqueue, leading to a double dequeue issue.

The double dequeue issue can occur in the following scenario:
1. Core0 call stack:
        autoremove_wake_function
        default_wake_function
        try_to_wake_up
        ttwu_do_activate
        task_woken_rt
        push_rt_task
        move_queued_task_locked
        dequeue_task
        __wake_up

2. Execution flow on Core0, Core1 and Core2(Core0, Core1 and Core2 are
   contending for Core1's rq->lock):
   - Core1: enqueue next_task on Core1
   - Core0: lock Core1's rq->lock
            next_task = pick_next_pushable_task()
            unlock Core1's rq->lock via double_lock_balance()
   - Core1: lock Core1's rq->lock
            next_task = pick_next_task()
            unlock Core1's rq->lock
   - Core2: lock Core1's rq->lock in migration thread
   - Core1: running next_task
   - Core2: unlock Core1's rq->lock
   - Core1: lock Core1's rq->lock
            switches out and dequeue next_task
            unlock Core1's rq->lock
   - Core0: relock Core1's rq->lock from double_lock_balance()
            try to relock Core1's rq->lock from double_lock_balance()
            but next_task has been dequeued from Core1, causing the issue

Signed-off-by: Tengfei Fan <tengfei.fan@xxxxxxxxxxxxxxxx>
---
 kernel/sched/rt.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 7936d4333731..b4e44317a5de 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2037,6 +2037,14 @@ static int push_rt_task(struct rq *rq, bool pull)
 		goto retry;
 	}
 
+	/* Within find_lock_lowest_rq(), it's possible to first unlock the
+	 * rq->lock of the runqueue containing next_task, and the re->lock
+	 * it. During this window, the state of next_task might have change.
+	 */
+	if (unlikely(rq != task_rq(next_task) ||
+		     !task_on_rq_queued(next_task)))
+		goto out;
+
 	move_queued_task_locked(rq, lowest_rq, next_task);
 	resched_curr(lowest_rq);
 	ret = 1;

---
base-commit: 7c3ba4249a3604477ea9c077e10089ba7ddcaa03
change-id: 20251008-recheck_rt_task_enqueue_state-e159aa6a2749

Best regards,
-- 
Tengfei Fan <tengfei.fan@xxxxxxxxxxxxxxxx>