[PATCH] sbitmap: Avoid leaving waitqueue in invalid state in __sbq_wake_up()

[Jan Kara <jack@suse.cz>]

When __sbq_wake_up() decrements wait_cnt to 0 but races with someone
else waking the waiter on the waitqueue (so the waitqueue becomes
empty), it exits without reseting wait_cnt to wake_batch number. Once
wait_cnt is 0, nobody will ever reset the wait_cnt or wake the new
waiters resulting in possible deadlocks or busyloops. Fix the problem by
making sure we reset wait_cnt even if we didn't wake up anybody in the
end.

Fixes: 040b83fcecfb ("sbitmap: fix possible io hung due to lost wakeup")
Reported-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 lib/sbitmap.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index a39b1a877366..47cd8fb894ba 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -604,6 +604,7 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
 	struct sbq_wait_state *ws;
 	unsigned int wake_batch;
 	int wait_cnt;
+	bool ret;
 
 	ws = sbq_wake_ptr(sbq);
 	if (!ws)
@@ -614,12 +615,23 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
 	 * For concurrent callers of this, callers should call this function
 	 * again to wakeup a new batch on a different 'ws'.
 	 */
-	if (wait_cnt < 0 || !waitqueue_active(&ws->wait))
+	if (wait_cnt < 0)
 		return true;
 
+	/*
+	 * If we decremented queue without waiters, retry to avoid lost
+	 * wakeups.
+	 */
 	if (wait_cnt > 0)
-		return false;
+		return !waitqueue_active(&ws->wait);
 
+	/*
+	 * When wait_cnt == 0, we have to be particularly careful as we are
+	 * responsible to reset wait_cnt regardless whether we've actually
+	 * woken up anybody. But in case we didn't wakeup anybody, we still
+	 * need to retry.
+	 */
+	ret = !waitqueue_active(&ws->wait);
 	wake_batch = READ_ONCE(sbq->wake_batch);
 
 	/*
@@ -648,7 +660,7 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
 	sbq_index_atomic_inc(&sbq->wake_index);
 	atomic_set(&ws->wait_cnt, wake_batch);
 
-	return false;
+	return ret;
 }
 
 void sbitmap_queue_wake_up(struct sbitmap_queue *sbq)
-- 
2.35.3