[PATCH sched_ext/for-6.19] sched_ext: Fix scx_bpf_dsq_peek() with FIFO DSQs

[Andrea Righi <arighi@nvidia.com>]

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 <arighi@nvidia.com>
---
 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)
-- 
2.51.1