[PATCH RFC v2 2/2] sbitmap: fix possible io hung due to lost wakeups

[Yu Kuai]

For example, 2 * wake_batch tags are put, while only wake_batch threads
are woken:

__sbq_wake_up
 atomic_cmpxchg -> reset wait_cnt
			__sbq_wake_up -> decrease wait_cnt
			...
			__sbq_wake_up -> wait_cnt is decreased to 0 again
			 atomic_cmpxchg
			 sbq_index_atomic_inc -> increase wake_index
			 wake_up_nr -> wake up and waitqueue might be empty
 sbq_index_atomic_inc -> increase again, one waitqueue is skipped
 wake_up_nr -> invalid wake up because old wakequeue might be empty

To fix the problem, increasing 'wake_index' before resetting 'wait_cnt'.

Fixes: 88459642cba4 ("blk-mq: abstract tag allocation out into sbitmap library")
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 lib/sbitmap.c | 43 ++++++++++++++++++++-----------------------
 1 file changed, 20 insertions(+), 23 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 4a230e5baacf..00ddb6593eff 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -615,34 +615,31 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
 		return false;
 
 	wait_cnt = atomic_dec_return(&ws->wait_cnt);
-	if (wait_cnt <= 0) {
-		int ret;
+	if (wait_cnt > 0)
+		return false;
 
-		wake_batch = READ_ONCE(sbq->wake_batch);
+	/*
+	 * For concurrent callers of this, callers should call this function
+	 * again to wakeup a new batch on a different 'ws'.
+	 */
+	if (wait_cnt < 0)
+		return true;
 
-		/*
-		 * Pairs with the memory barrier in sbitmap_queue_resize() to
-		 * ensure that we see the batch size update before the wait
-		 * count is reset.
-		 */
-		smp_mb__before_atomic();
+	wake_batch = READ_ONCE(sbq->wake_batch);
 
-		/*
-		 * For concurrent callers of this, the one that failed the
-		 * atomic_cmpxhcg() race should call this function again
-		 * to wakeup a new batch on a different 'ws'.
-		 */
-		ret = atomic_cmpxchg(&ws->wait_cnt, wait_cnt, wake_batch);
-		if (ret == wait_cnt) {
-			sbq_index_atomic_inc(&sbq->wake_index);
-			wake_up_nr(&ws->wait, wake_batch);
-			return false;
-		}
+	/*
+	 * Pairs with the memory barrier in sbitmap_queue_resize() to
+	 * ensure that we see the batch size update before the wait
+	 * count is reset.
+	 */
+	smp_mb__before_atomic();
 
-		return true;
-	}
+	/* increase wake_index first to prevent possible lost wakeups */
+	sbq_index_atomic_inc(&sbq->wake_index);
+	atomic_set(&ws->wait_cnt, wake_batch);
+	wake_up_nr(&ws->wait, wake_batch);
 
-	return false;
+	return true;
 }
 
 void sbitmap_queue_wake_up(struct sbitmap_queue *sbq)
-- 
2.31.1