blk-mq: fix potential hang if rolling wakeup depth is too high

We currently divide the queue depth by 4 as our batch wakeup
count, but we split the wakeups over BT_WAIT_QUEUES number of
wait queues. This defaults to 8. If the product of the resulting
batch wake count and BT_WAIT_QUEUES is higher than the device
queue depth, we can get into a situation where a task goes to
sleep waiting for a request, but never gets woken up.

Reported-by: Bart Van Assche <bvanassche@acm.org>
Fixes: 4bb659b156996
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index b08788086414..146fd02659ec 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -455,8 +455,8 @@ static void bt_update_count(struct blk_mq_bitmap_tags *bt,
         }
 
         bt->wake_cnt = BT_WAIT_BATCH;
-        if (bt->wake_cnt > depth / 4)
-                bt->wake_cnt = max(1U, depth / 4);
+        if (bt->wake_cnt > depth / BT_WAIT_QUEUES)
+                bt->wake_cnt = max(1U, depth / BT_WAIT_QUEUES);
 
         bt->depth = depth;
 }