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

[Keith Busch <kbusch@kernel.org>]

On Mon, Feb 09, 2026 at 08:40:29AM -0800, Bart Van Assche wrote:
> On 2/9/26 12:29 AM, 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.
> 
> The above looks fine to me.
> 
> > @@ -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);
> >   }
> 
> This is not clear to me. Why are concurrent queue freezes not allowed
> while debugfs attributes are created? I don't see any code in debugfs
> that calls back into the block layer while creating debugfs attributes?
> Did I perhaps overlook something?

I had to look up the original commit that introduced the WARN,
65d466b629847. The commit message says "Creating new debugfs entries can
trigger fs reclaim", so that must be the path that enters back into the
blaock layer request_queue.