On 11/17/25 4:31 PM, Ming Lei wrote:

On Sun, Nov 16, 2025 at 12:10:20PM +0800, Yu Kuai wrote:

quoted

queue should not be freezed under rq_qos_mutex, see example index
commit 9730763f4756 ("block: correct locking order for protecting blk-wbt
parameters"), which means current implementation of rq_qos_add() is
problematic. Add a new helper and prepare to fix this problem in
following patches.

Signed-off-by: Yu Kuai <redacted>
---
 block/blk-rq-qos.c | 27 +++++++++++++++++++++++++++
 block/blk-rq-qos.h |  2 ++
 2 files changed, 29 insertions(+)

diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
index 654478dfbc20..353397d7e126 100644
--- a/block/blk-rq-qos.c
+++ b/block/blk-rq-qos.c

@@ -322,6 +322,33 @@ void rq_qos_exit(struct request_queue *q)
 	mutex_unlock(&q->rq_qos_mutex);
 }
 
+int rq_qos_add_freezed(struct rq_qos *rqos, struct gendisk *disk,
+		       enum rq_qos_id id, const struct rq_qos_ops *ops)
+{
+	struct request_queue *q = disk->queue;
+
+	WARN_ON_ONCE(q->mq_freeze_depth == 0);
+	lockdep_assert_held(&q->rq_qos_mutex);
+
+	if (rq_qos_id(q, id))
+		return -EBUSY;
+
+	rqos->disk = disk;
+	rqos->id = id;
+	rqos->ops = ops;
+	rqos->next = q->rq_qos;
+	q->rq_qos = rqos;
+	blk_queue_flag_set(QUEUE_FLAG_QOS_ENABLED, q);
+
+	if (rqos->ops->debugfs_attrs) {
+		mutex_lock(&q->debugfs_mutex);
+		blk_mq_debugfs_register_rqos(rqos);
+		mutex_unlock(&q->debugfs_mutex);
+	}

It will cause more lockdep splat to let q->debugfs_mutex depend on queue freeze,

I think we already have that ->debugfs_mutex dependency on ->freeze_lock. 
for instance, 
  ioc_qos_write  => freeze-queue
   blk_iocost_init
     rq_qos_add 

and also, 
  queue_wb_lat_store  => freeze-queue
    wbt_init
      rq_qos_add

Also blk_mq_debugfs_register_rqos() does _not_ require queue to be frozen,
and it should be fine to move blk_mq_debugfs_register_rqos() out of queue
freeze.

Yes correct, but I thought this pacthset is meant only to address incorrect 
locking order between ->rq_qos_mutex and ->freeze_lock. So do you suggest
also refactoring code to avoid ->debugfs_mutex dependency on ->freeze_lock?
If yes then shouldn't that be handled in a separate patchset?

Thanks,
--Nilay