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

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

On Mon, Nov 17, 2025 at 04:43:11PM +0530, Nilay Shroff wrote:
> 
> 
> On 11/17/25 4:31 PM, Ming Lei wrote:
> > 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,
> > 
> 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 

Why is queue freeze needed in above code path?

Also blk_iolatency_init()/rq_qos_add() doesn't freeze queue.

> 
> and also, 
>   queue_wb_lat_store  => freeze-queue
>     wbt_init
>       rq_qos_add

Not all wbt_enable_default()/wbt_init() is called with queue frozen, but Kuai's
patchset changes all to freeze queue before registering debugfs entry, people will
complain new warning.

> 
> > 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?

It is fine to fix in that way, but at least regression shouldn't be caused.

More importantly we shouldn't add new unnecessary dependency on queue freeze.

Thanks,
Ming