[PATCH 2/2] block: clear QUEUE_FLAG_QOS_ENABLED in rq_qos_del()

[Nilay Shroff <nilay@linux.ibm.com>]

When a QoS function is removed via rq_qos_del(), and it happens to be the
last QoS function on the request queue, q->rq_qos becomes NULL. In this
case, the QUEUE_FLAG_QOS_ENABLED bit should also be cleared to reflect
that no QoS hooks remain active.

This patch ensures that the QUEUE_FLAG_QOS_ENABLED flag is cleared if the
queue no longer has any associated rq_qos policies. Failing to do so
could cause unnecessary dereferences of a now-null q->rq_qos pointer in
the I/O path.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 block/blk-rq-qos.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
index 460c04715321..654478dfbc20 100644
--- a/block/blk-rq-qos.c
+++ b/block/blk-rq-qos.c
@@ -375,6 +375,8 @@ void rq_qos_del(struct rq_qos *rqos)
 			break;
 		}
 	}
+	if (!q->rq_qos)
+		blk_queue_flag_clear(QUEUE_FLAG_QOS_ENABLED, q);
 	blk_mq_unfreeze_queue(q, memflags);
 
 	mutex_lock(&q->debugfs_mutex);
-- 
2.50.1