Re: [PATCH RESEND 1/5] block/blk-rq-qos: add a new helper rq_qos_add_freezed()

[Ming Lei <ming.lei@redhat.com>]

On Sun, Nov 16, 2025 at 12:10:20PM +0800, Yu Kuai wrote:
> 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 <yukuai@fnnas.com>
> ---
>  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,

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.


Thanks,
Ming