[PATCH v9 4/8] blk-rq-qos: fix possible debugfs_mutex deadlock

[Yu Kuai <yukuai@fnnas.com>]

Currently rq-qos debugfs entries are created from rq_qos_add(), while
rq_qos_add() can be called while queue is still frozen. This can
deadlock because creating new entries can trigger fs reclaim.

Fix this problem by delaying creating rq-qos debugfs entries after queue
is unfrozen.

- For wbt, 1) it can be initialized by default, fix it by calling new
  helper after wbt_init() from wbt_init_enable_default(); 2) it can be
  initialized by sysfs, fix it by calling new helper after queue is
  unfrozen from wbt_set_lat().
- For iocost and iolatency, they can only be initialized by blkcg
  configuration, however, they don't have debugfs entries for now, hence
  they are not handled yet.

Signed-off-by: Yu Kuai <yukuai@fnnas.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
---
 block/blk-rq-qos.c |  7 -------
 block/blk-wbt.c    | 13 ++++++++++++-
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
index 654478dfbc20..d7ce99ce2e80 100644
--- a/block/blk-rq-qos.c
+++ b/block/blk-rq-qos.c
@@ -347,13 +347,6 @@ int rq_qos_add(struct rq_qos *rqos, struct gendisk *disk, enum rq_qos_id id,
 	blk_queue_flag_set(QUEUE_FLAG_QOS_ENABLED, q);
 
 	blk_mq_unfreeze_queue(q, memflags);
-
-	if (rqos->ops->debugfs_attrs) {
-		mutex_lock(&q->debugfs_mutex);
-		blk_mq_debugfs_register_rqos(rqos);
-		mutex_unlock(&q->debugfs_mutex);
-	}
-
 	return 0;
 ebusy:
 	blk_mq_unfreeze_queue(q, memflags);
diff --git a/block/blk-wbt.c b/block/blk-wbt.c
index 665760274e60..1415f2bf8611 100644
--- a/block/blk-wbt.c
+++ b/block/blk-wbt.c
@@ -774,6 +774,7 @@ EXPORT_SYMBOL_GPL(wbt_enable_default);
 
 void wbt_init_enable_default(struct gendisk *disk)
 {
+	struct request_queue *q = disk->queue;
 	struct rq_wb *rwb;
 
 	if (!__wbt_enable_default(disk))
@@ -783,8 +784,14 @@ void wbt_init_enable_default(struct gendisk *disk)
 	if (WARN_ON_ONCE(!rwb))
 		return;
 
-	if (WARN_ON_ONCE(wbt_init(disk, rwb)))
+	if (WARN_ON_ONCE(wbt_init(disk, rwb))) {
 		wbt_free(rwb);
+		return;
+	}
+
+	mutex_lock(&q->debugfs_mutex);
+	blk_mq_debugfs_register_rq_qos(q);
+	mutex_unlock(&q->debugfs_mutex);
 }
 
 static u64 wbt_default_latency_nsec(struct request_queue *q)
@@ -1008,5 +1015,9 @@ int wbt_set_lat(struct gendisk *disk, s64 val)
 	blk_mq_unquiesce_queue(q);
 out:
 	blk_mq_unfreeze_queue(q, memflags);
+	mutex_lock(&q->debugfs_mutex);
+	blk_mq_debugfs_register_rq_qos(q);
+	mutex_unlock(&q->debugfs_mutex);
+
 	return ret;
 }
-- 
2.51.0