[PATCH 3.16.y-ckt 108/254] blk-mq: fix potential hang if rolling wakeup depth is too high

[Luis Henriques]

3.16.7-ckt2 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jens Axboe <axboe at fb.com>

commit abab13b5c4fd1fec4f9a61622548012d93dc2831 upstream.

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 at acm.org>
Fixes: 4bb659b156996
Signed-off-by: Jens Axboe <axboe at fb.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 block/blk-mq-tag.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index c1b92426c95e..74a4168ea34e 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -463,8 +463,8 @@ 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;
 }
-- 
2.1.0