linux-kernel - Re: [PATCH 1/4] blk-mq-debugfs: warn about possible deadlock

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Message-ID: <aO4EniFy63IlWM_-@fedora>
Date: Tue, 14 Oct 2025 16:06:54 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Yu Kuai <yukuai3@...wei.com>
Cc: nilay@...ux.ibm.com, tj@...nel.org, josef@...icpanda.com,
	axboe@...nel.dk, cgroups@...r.kernel.org,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	yukuai1@...weicloud.com, yi.zhang@...wei.com, yangerkun@...wei.com,
	johnny.chenyi@...wei.com
Subject: Re: [PATCH 1/4] blk-mq-debugfs: warn about possible deadlock

On Tue, Oct 14, 2025 at 10:21:46AM +0800, Yu Kuai wrote:
> Creating new debugfs entries can trigger fs reclaim, hence we can't do
> this with queue freezed, meanwhile, other locks that can be held while
> queue is freezed should not be held as well.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>  block/blk-mq-debugfs.c | 31 ++++++++++++++++++++++++-------
>  1 file changed, 24 insertions(+), 7 deletions(-)
> 
> diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
> index 4896525b1c05..66864ed0b77f 100644
> --- a/block/blk-mq-debugfs.c
> +++ b/block/blk-mq-debugfs.c
> @@ -608,9 +608,23 @@ static const struct blk_mq_debugfs_attr blk_mq_debugfs_ctx_attrs[] = {
>  	{},
>  };
>  
> -static void debugfs_create_files(struct dentry *parent, void *data,
> +static void debugfs_create_files(struct request_queue *q, struct dentry *parent,
> +				 void *data,
>  				 const struct blk_mq_debugfs_attr *attr)
>  {
> +	/*
> +	 * Creating new debugfs entries with queue freezed has the rist 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 freezed.
> +	 */
> +	lockdep_assert_not_held(&q->elevator_lock);
> +	lockdep_assert_not_held(&q->rq_qos_mutex);

->rq_qos_mutex use looks one real mess, in blk-cgroup.c, it is grabbed after
queue is frozen. However, inside block/blk-rq-qos.c, the two are re-ordered,
maybe we need to fix order between queue freeze and q->rq_qos_mutex first?
Or move on by removing the above line?

Otherwise, this patch looks good.


Thanks,
Ming