From 71d7847cad4475f1f795c7737e08b604b448ca70 Mon Sep 17 00:00:00 2001 From: Andrea Righi Date: Sat, 25 Oct 2025 00:01:02 +0200 Subject: [PATCH] sched_ext: Fix scx_bpf_dsq_peek() with FIFO DSQs When removing a task from a FIFO DSQ, we must delete it from the list before updating dsq->first_task, otherwise the following lookup will just re-read the same task, leaving first_task pointing to removed entry. This issue only affects DSQs operating in FIFO mode, as priority DSQs correctly update the rbtree before re-evaluating the new first task. Remove the item from the list before refreshing the first task to guarantee the correct behavior in FIFO DSQs. Fixes: 44f5c8ec5b9ad ("sched_ext: Add lockless peek operation for DSQs") Signed-off-by: Andrea Righi Signed-off-by: Tejun Heo --- kernel/sched/ext.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index bde49e47f1e4b..1cfe4b43d31b0 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -1044,15 +1044,15 @@ static void task_unlink_from_dsq(struct task_struct *p, p->scx.dsq_flags &= ~SCX_TASK_DSQ_ON_PRIQ; } + list_del_init(&p->scx.dsq_list.node); + dsq_mod_nr(dsq, -1); + if (!(dsq->id & SCX_DSQ_FLAG_BUILTIN) && dsq->first_task == p) { struct task_struct *first_task; first_task = nldsq_next_task(dsq, NULL, false); rcu_assign_pointer(dsq->first_task, first_task); } - - list_del_init(&p->scx.dsq_list.node); - dsq_mod_nr(dsq, -1); } static void dispatch_dequeue(struct rq *rq, struct task_struct *p) -- 2.47.3