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

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

On Thu, Aug 14, 2025 at 08:01:11PM +0530, Nilay Shroff wrote:
> 
> 
> 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?

But it is that in-tree code is doing, isn't it?

`static branch` is only evaluated iff at least one rqos is added.


Thanks, 
Ming