Re: [PATCH v2] sched_ext: Don't BUG_ON a destroyed DSQ in process_deferred_reenq_users

[Tejun Heo]

Hello,

Applied the following to sched_ext/for-7.3. Thanks.

-- 8< --
From: Tao Cui <cuitao@xxxxxxxxxx>
Date: Sat, 15 Aug 2026 10:20:17 +0800
Subject: [PATCH] sched_ext: Don't BUG_ON a destroyed DSQ in
 process_deferred_reenq_users

scx_bpf_dsq_reenq() queues a deferred reenq (dru) that runs from
run_deferred(), not ops.dispatch(). If the DSQ is destroyed before the dru
runs, process_deferred_reenq_users() sees dsq->id == SCX_DSQ_INVALID and
hits the BUG_ON. destroy_dsq() doesn't flush pending drus, so just skip.

tj: Read dsq->id once with READ_ONCE(). Reading it separately in the INVALID
    check and the BUG_ON would leave a window where destroy_dsq() can
    invalidate the id between the two reads and still trigger the BUG_ON.

Fixes: 84b1a0ea0b7c ("sched_ext: Implement scx_bpf_dsq_reenq() for user DSQs")
Cc: stable@xxxxxxxxxxxxxxx # v7.1+
Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
 kernel/sched/ext/ext.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index b9060c318c3c..7620ebf02790 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -4613,7 +4613,7 @@ static void process_deferred_reenq_users(struct rq *rq)
 
 	while (true) {
 		struct scx_dispatch_q *dsq;
-		u64 reenq_flags;
+		u64 dsq_id, reenq_flags;
 
 		scoped_guard (raw_spinlock, &rq->scx.deferred_reenq_lock) {
 			struct scx_deferred_reenq_user *dru =
@@ -4636,7 +4636,12 @@ static void process_deferred_reenq_users(struct rq *rq)
 		/* see schedule_dsq_reenq() */
 		smp_mb();
 
-		BUG_ON(dsq->id & SCX_DSQ_FLAG_BUILTIN);
+		/* destroy_dsq() may have raced and invalidated @dsq, nothing to reenq */
+		dsq_id = READ_ONCE(dsq->id);
+		if (unlikely(dsq_id == SCX_DSQ_INVALID))
+			continue;
+
+		BUG_ON(dsq_id & SCX_DSQ_FLAG_BUILTIN);
 		reenq_user(rq, dsq, reenq_flags);
 	}
 }
-- 
2.55.0