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 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.



Thanks,
Ming