linux-kernel - [PATCH 2/5] blk-wbt: fix incorrect lock order for rq_qos

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Message-Id: <20251116024134.115685-3-yukuai@fnnas.com>
Date: Sun, 16 Nov 2025 10:41:31 +0800
From: "Yu Kuai" <yukuai@...as.com>
To: <axboe@...nel.dk>, <linux-block@...r.kernel.org>, 
	<linux-kernel@...r.kernel.org>, <tj@...nel.org>, <nilay@...ux.ibm.com>, 
	<ming.lei@...hat.com>
Cc: <yukuai@...as.com>
Subject: [PATCH 2/5] blk-wbt: fix incorrect lock order for rq_qos_mutex and freeze queue

wbt_init() can be called from sysfs attribute and wbt_enable_default(),
however the lock order are inversely.

- queue_wb_lat_store() freeze queue first, and then wbt_init() hold
  rq_qos_mutex. In this case queue will be freezed again inside
  rq_qos_add(), however, in this case freeze queue recursivly is
  inoperative;
- wbt_enable_default() from elevator switch will hold rq_qos_mutex
  first, and then rq_qos_add() will freeze queue;

Fix this problem by converting to use new helper rq_qos_add_freezed() in
wbt_init(), and for wbt_enable_default(), freeze queue before calling
wbt_init().

Fixes: a13bd91be223 ("block/rq_qos: protect rq_qos apis with a new lock")
Signed-off-by: Yu Kuai <yukuai@...as.com>
---
 block/blk-wbt.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/block/blk-wbt.c b/block/blk-wbt.c
index eb8037bae0bd..a784f6d338b4 100644
--- a/block/blk-wbt.c
+++ b/block/blk-wbt.c
@@ -724,8 +724,12 @@ void wbt_enable_default(struct gendisk *disk)
 	if (!blk_queue_registered(q))
 		return;
 
-	if (queue_is_mq(q) && enable)
+	if (queue_is_mq(q) && enable) {
+		unsigned int memflags = blk_mq_freeze_queue(q);
+
 		wbt_init(disk);
+		blk_mq_unfreeze_queue(q, memflags);
+	}
 }
 EXPORT_SYMBOL_GPL(wbt_enable_default);
 
@@ -922,7 +926,7 @@ int wbt_init(struct gendisk *disk)
 	 * Assign rwb and add the stats callback.
 	 */
 	mutex_lock(&q->rq_qos_mutex);
-	ret = rq_qos_add(&rwb->rqos, disk, RQ_QOS_WBT, &wbt_rqos_ops);
+	ret = rq_qos_add_freezed(&rwb->rqos, disk, RQ_QOS_WBT, &wbt_rqos_ops);
 	mutex_unlock(&q->rq_qos_mutex);
 	if (ret)
 		goto err_free;
-- 
2.51.0