Re: [PATCHv3 3/3] block: avoid cpu_hotplug_lock depedency on freeze_lock

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

On 8/14/25 7:08 PM, Ming Lei wrote:
> On Thu, Aug 14, 2025 at 06:27:08PM +0530, Nilay Shroff wrote:
>>
>>
>> On 8/14/25 6:14 PM, Ming Lei wrote:
>>> On Thu, Aug 14, 2025 at 01:54:59PM +0530, Nilay Shroff wrote:
>>>> A recent lockdep[1] splat observed while running blktest block/005
>>>> reveals a potential deadlock caused by the cpu_hotplug_lock dependency
>>>> on ->freeze_lock. This dependency was introduced by commit 033b667a823e
>>>> ("block: blk-rq-qos: guard rq-qos helpers by static key").
>>>>
>>>> That change added a static key to avoid fetching q->rq_qos when
>>>> neither blk-wbt nor blk-iolatency is configured. The static key
>>>> dynamically patches kernel text to a NOP when disabled, eliminating
>>>> overhead of fetching q->rq_qos in the I/O hot path. However, enabling
>>>> a static key at runtime requires acquiring both cpu_hotplug_lock and
>>>> jump_label_mutex. When this happens after the queue has already been
>>>> frozen (i.e., while holding ->freeze_lock), it creates a locking
>>>> dependency from cpu_hotplug_lock to ->freeze_lock, which leads to a
>>>> potential deadlock reported by lockdep [1].
>>>>
>>>> To resolve this, replace the static key mechanism with q->queue_flags:
>>>> QUEUE_FLAG_QOS_ENABLED. This flag is evaluated in the fast path before
>>>> accessing q->rq_qos. If the flag is set, we proceed to fetch q->rq_qos;
>>>> otherwise, the access is skipped.
>>>>
>>>> Since q->queue_flags is commonly accessed in IO hotpath and resides in
>>>> the first cacheline of struct request_queue, checking it imposes minimal
>>>> overhead while eliminating the deadlock risk.
>>>>
>>>> This change avoids the lockdep splat without introducing performance
>>>> regressions.
>>>>
>>>> [1] https://lore.kernel.org/linux-block/4fdm37so3o4xricdgfosgmohn63aa7wj3ua4e5vpihoamwg3ui@fq42f5q5t5ic/
>>>>
>>>> Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
>>>> Closes: https://lore.kernel.org/linux-block/4fdm37so3o4xricdgfosgmohn63aa7wj3ua4e5vpihoamwg3ui@fq42f5q5t5ic/
>>>> Fixes: 033b667a823e ("block: blk-rq-qos: guard rq-qos helpers by static key")
>>>> Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
>>>> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
>>>> ---
>>>>  block/blk-mq-debugfs.c |  1 +
>>>>  block/blk-rq-qos.c     |  9 ++++---
>>>>  block/blk-rq-qos.h     | 54 ++++++++++++++++++++++++------------------
>>>>  include/linux/blkdev.h |  1 +
>>>>  4 files changed, 37 insertions(+), 28 deletions(-)
>>>>
>>>> diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
>>>> index 7ed3e71f2fc0..32c65efdda46 100644
>>>> --- a/block/blk-mq-debugfs.c
>>>> +++ b/block/blk-mq-debugfs.c
>>>> @@ -95,6 +95,7 @@ static const char *const blk_queue_flag_name[] = {
>>>>  	QUEUE_FLAG_NAME(SQ_SCHED),
>>>>  	QUEUE_FLAG_NAME(DISABLE_WBT_DEF),
>>>>  	QUEUE_FLAG_NAME(NO_ELV_SWITCH),
>>>> +	QUEUE_FLAG_NAME(QOS_ENABLED),
>>>>  };
>>>>  #undef QUEUE_FLAG_NAME
>>>>  
>>>> diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
>>>> index b1e24bb85ad2..654478dfbc20 100644
>>>> --- a/block/blk-rq-qos.c
>>>> +++ b/block/blk-rq-qos.c
>>>> @@ -2,8 +2,6 @@
>>>>  
>>>>  #include "blk-rq-qos.h"
>>>>  
>>>> -__read_mostly DEFINE_STATIC_KEY_FALSE(block_rq_qos);
>>>> -
>>>>  /*
>>>>   * Increment 'v', if 'v' is below 'below'. Returns true if we succeeded,
>>>>   * false if 'v' + 1 would be bigger than 'below'.
>>>> @@ -319,8 +317,8 @@ void rq_qos_exit(struct request_queue *q)
>>>>  		struct rq_qos *rqos = q->rq_qos;
>>>>  		q->rq_qos = rqos->next;
>>>>  		rqos->ops->exit(rqos);
>>>> -		static_branch_dec(&block_rq_qos);
>>>>  	}
>>>> +	blk_queue_flag_clear(QUEUE_FLAG_QOS_ENABLED, q);
>>>>  	mutex_unlock(&q->rq_qos_mutex);
>>>>  }
>>>>  
>>>> @@ -346,7 +344,7 @@ int rq_qos_add(struct rq_qos *rqos, struct gendisk *disk, enum rq_qos_id id,
>>>>  		goto ebusy;
>>>>  	rqos->next = q->rq_qos;
>>>>  	q->rq_qos = rqos;
>>>> -	static_branch_inc(&block_rq_qos);
>>>> +	blk_queue_flag_set(QUEUE_FLAG_QOS_ENABLED, q);
>>>
>>> One stupid question: can we simply move static_branch_inc(&block_rq_qos)
>>> out of queue freeze in rq_qos_add()?
>>>
>>> What matters is just the 1st static_branch_inc() which switches the counter
>>> from 0 to 1, when blk_mq_freeze_queue() guarantees that all in-progress code
>>> paths observe q->rq_qos as NULL. That means static_branch_inc(&block_rq_qos)
>>> needn't queue freeze protection.
>>>
>> I thought about it earlier but that won't work because we have 
>> code paths freezing queue before it reaches upto rq_qos_add(),
>> For instance:
>>
>> We have following code paths from where we invoke
>> rq_qos_add() APIs with queue already frozen:
>>
>> ioc_qos_write()
>>  -> blkg_conf_open_bdev_frozen() => freezes queue
>>  -> blk_iocost_init()
>>    -> rq_qos_add()
>>
>> queue_wb_lat_store()  => freezes queue
>>  -> wbt_init()
>>   -> rq_qos_add() 
> 
> The above two shouldn't be hard to solve, such as, add helper
> rq_qos_prep_add() for increasing the static branch counter.
> 
Yes but then it means that IOs which would be in flight 
would take a hit in hotpath: In hotpath those IOs
would evaluate static key branch to true and then fetch 
q->rq_qos (which probably would not be in the first
cacheline). So are we okay to take hat hit in IO 
hotpath?

Thanks,
--Nilay