[PATCH 4/4] blk-mq: use blk_queue_enter/exit to protect debugfs file creation

[Nilay Shroff]

On 2/9/26 1:59 PM, Yu Kuai wrote:
> diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
> index faeaa1fc86a7..03583d0d3972 100644
> --- a/block/blk-mq-debugfs.c
> +++ b/block/blk-mq-debugfs.c
> @@ -613,11 +613,6 @@ static void debugfs_create_files(struct request_queue *q, struct dentry *parent,
>  				 const struct blk_mq_debugfs_attr *attr)
>  {
>  	lockdep_assert_held(&q->debugfs_mutex);
> -	/*
> -	 * Creating new debugfs entries with queue freezed has the risk of
> -	 * deadlock.
> -	 */
> -	WARN_ON_ONCE(q->mq_freeze_depth != 0);
>  	/*
>  	 * debugfs_mutex should not be nested under other locks that can be
>  	 * grabbed while queue is frozen.
> @@ -628,9 +623,19 @@ static void debugfs_create_files(struct request_queue *q, struct dentry *parent,
>  	if (IS_ERR_OR_NULL(parent))
>  		return;
>  
> +	/*
> +	 * Avoid creating debugfs files while the queue is frozen, wait for
> +	 * the queue to be unfrozen and prevent new freeze while creating
> +	 * debugfs files.
> +	 */
> +	if (blk_queue_enter(q, 0))
> +		return;
> +
>  	for (; attr->name; attr++)
>  		debugfs_create_file_aux(attr->name, attr->mode, parent,
>  				    (void *)attr, data, &blk_mq_debugfs_fops);
> +
> +	blk_queue_exit(q);
>  }
>  
>  void blk_mq_debugfs_register(struct request_queue *q)

I also noticed that we use other debugfs helpers such as debugfs_create_dir()
from some block paths, which could run into the same fs-reclaim recursion issue.
So we may need to consider those call sites as well to ensure consistent handling.

On another note, instead of waiting for queue to be unfreezed, does it makes sense to
wrap the debugfs code under NOIO context (as Christoph pointed in another thread)?
IMO, if fs-reclaim recursion is the only issue here we would be better off using
NOIO context instead of blocking for queue to be unfrozen from other contexts.

Thanks,
--Nilay