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

[Nilay Shroff <nilay@linux.ibm.com>]

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 

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