[PATCH] sched/wait: avoid modifying entry on failed priority registration

[Hui Su <sh_def@163.com>]

add_wait_queue_priority_exclusive() sets WQ_FLAG_EXCLUSIVE and
WQ_FLAG_PRIORITY before checking whether another priority waiter is
already present. Thus, when registration fails with -EBUSY, the entry
is left unqueued but with modified flags.

There is no need to modify the entry before the failure check. Move the
flag update after the check so that a rejected registration does not
unnecessarily alter the entry.

Signed-off-by: Hui Su <sh_def@163.com>

---
Testing:
- Private x86_64/QEMU differential oracle: the old path modified the entry
  flags on -EBUSY; the patched path preserved them.
---
 kernel/sched/wait.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
index d033f600f48c..93b353fbd0c9 100644
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -54,14 +54,13 @@ int add_wait_queue_priority_exclusive(struct wait_queue_head *wq_head,
 {
 	struct list_head *head = &wq_head->head;
 
-	wq_entry->flags |= WQ_FLAG_EXCLUSIVE | WQ_FLAG_PRIORITY;
-
 	guard(spinlock_irqsave)(&wq_head->lock);
 
 	if (!list_empty(head) &&
 	    (list_first_entry(head, typeof(*wq_entry), entry)->flags & WQ_FLAG_PRIORITY))
 		return -EBUSY;
 
+	wq_entry->flags |= WQ_FLAG_EXCLUSIVE | WQ_FLAG_PRIORITY;
 	list_add(&wq_entry->entry, head);
 	return 0;
 }

base-commit: 3857c2fe5449541e24afc5efdb0f81a8a8f9a3a0