[PATCH v7 4/5] blk-mq: Make it safe to use RCU to iterate over blk_mq_tag_set.tag_list

[Bart Van Assche <bvanassche@acm.org>]

Since the next patch in this series will use RCU to iterate over tag_list,
make this safe. Note: call_rcu() is already used to free the request queue.
From blk-sysfs.c:

	call_rcu(&q->rcu_head, blk_free_queue_rcu);

See also:
* Commit 705cda97ee3a ("blk-mq: Make it safe to use RCU to iterate over
  blk_mq_tag_set.tag_list"; v4.12).
* Commit 08c875cbf481 ("block: Use non _rcu version of list functions for
  tag_set_list"; v5.9).

Reviewed-by: Khazhismel Kumykov <khazhy@google.com>
Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/blk-mq.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 8b59f6b4ec8e..7d2ea6357c7d 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2947,7 +2947,7 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q)
 	struct blk_mq_tag_set *set = q->tag_set;
 
 	mutex_lock(&set->tag_list_lock);
-	list_del(&q->tag_set_list);
+	list_del_rcu(&q->tag_set_list);
 	if (list_is_singular(&set->tag_list)) {
 		/* just transitioned to unshared */
 		set->flags &= ~BLK_MQ_F_TAG_QUEUE_SHARED;
@@ -2955,7 +2955,11 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q)
 		blk_mq_update_tag_set_shared(set, false);
 	}
 	mutex_unlock(&set->tag_list_lock);
-	INIT_LIST_HEAD(&q->tag_set_list);
+	/*
+	 * Calling synchronize_rcu() and INIT_LIST_HEAD(&q->tag_set_list) is
+	 * not necessary since blk_mq_del_queue_tag_set() is only called from
+	 * blk_cleanup_queue().
+	 */
 }
 
 static void blk_mq_add_queue_tag_set(struct blk_mq_tag_set *set,