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

[Tao Cui <cui.tao@linux.dev>]

From: Tao Cui <cuitao@kylinos.cn>

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.

Fixes: 84b1a0ea0b7c ("sched_ext: Implement scx_bpf_dsq_reenq() for user DSQs")
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
Change in v2:
Only skip SCX_DSQ_INVALID and keep the BUG_ON for other builtin ids
instead of skipping every builtin-flagged id, per Tejun.
---
 kernel/sched/ext/ext.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 26dbbbfe5a87..7d097209e778 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -4637,6 +4637,10 @@ static void process_deferred_reenq_users(struct rq *rq)
 		/* see schedule_dsq_reenq() */
 		smp_mb();
 
+		/* destroy_dsq() may have raced and invalidated @dsq, nothing to reenq */
+		if (unlikely(dsq->id == SCX_DSQ_INVALID))
+			continue;
+
 		BUG_ON(dsq->id & SCX_DSQ_FLAG_BUILTIN);
 		reenq_user(rq, dsq, reenq_flags);
 	}
-- 
2.43.0