Re: [PATCH next] sbitmap: fix lockup while swapping

Keith Busch
2022-09-23T19:07:30Z

Does the following fix the observation? Rational being that there's no reason
to spin on the current wait state that is already under handling; let
subsequent clearings proceed to the next inevitable wait state immediately.

---
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 624fa7f118d1..47bf7882210b 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -634,6 +634,13 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq, int *nr)

*nr -= sub;

+ /*
+ * Increase wake_index before updating wait_cnt, otherwise concurrent
+ * callers can see valid wait_cnt in old waitqueue, which can cause
+ * invalid wakeup on the old waitqueue.
+ */
+ sbq_index_atomic_inc(&sbq->wake_index);
+
/*
* When wait_cnt == 0, we have to be particularly careful as we are
* responsible to reset wait_cnt regardless whether we've actually
@@ -660,13 +667,6 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq, int *nr)
* of atomic_set().
*/
smp_mb__before_atomic();
-
- /*
- * Increase wake_index before updating wait_cnt, otherwise concurrent
- * callers can see valid wait_cnt in old waitqueue, which can cause
- * invalid wakeup on the old waitqueue.
- */
- sbq_index_atomic_inc(&sbq->wake_index);
atomic_set(&ws->wait_cnt, wake_batch);

return ret || *nr;
--