Re: [PATCH] nvme: Convert tag_list mutex to rwsemaphore to avoid deadlock

[Chaitanya Kulkarni <chaitanyak@nvidia.com>]

On 11/13/25 12:23, Mohamed Khalfella wrote:
> blk_mq_{add,del}_queue_tag_set() functions add and remove queues from
> tagset, the functions make sure that tagset and queues are marked as
> shared when two or more queues are attached to the same tagset.
> Initially a tagset starts as unshared and when the number of added
> queues reaches two, blk_mq_add_queue_tag_set() marks it as shared along
> with all the queues attached to it. When the number of attached queues
> drops to 1 blk_mq_del_queue_tag_set() need to mark both the tagset and
> the remaining queues as unshared.
>
> Both functions need to freeze current queues in tagset before setting on
> unsetting BLK_MQ_F_TAG_QUEUE_SHARED flag. While doing so, both functions
> hold set->tag_list_lock mutex, which makes sense as we do not want
> queues to be added or deleted in the process. This used to work fine
> until commit 98d81f0df70c ("nvme: use blk_mq_[un]quiesce_tagset")
> made the nvme driver quiesce tagset instead of quiscing individual
> queues. blk_mq_quiesce_tagset() does the job and quiesce the queues in
> set->tag_list while holding set->tag_list_lock also.
>
> This results in deadlock between two threads with these stacktraces:
>
[...]

>
> The top stacktrace is showing nvme_timeout() called to handle nvme
> command timeout. timeout handler is trying to disable the controller and
> as a first step, it needs to blk_mq_quiesce_tagset() to tell blk-mq not
> to call queue callback handlers. The thread is stuck waiting for
> set->tag_list_lock as it tires to walk the queues in set->tag_list.
>
> The lock is held by the second thread in the bottom stack which is
> waiting for one of queues to be frozen. The queue usage counter will
> drop to zero after nvme_timeout() finishes, and this will not happen
> because the thread will wait for this mutex forever.
>
> Convert set->tag_list_lock mutex to set->tag_list_rwsem rwsemaphore to
> avoid the deadlock. Update blk_mq_[un]quiesce_tagset() to take the
> semaphore for read since this is enough to guarantee no queues will be
> added or removed. Update blk_mq_{add,del}_queue_tag_set() to take the
> semaphore for write while updating set->tag_list and downgrade it to
> read while freezing the queues. It should be safe to update set->flags
> and hctx->flags while holding the semaphore for read since the queues
> are already frozen.
>
> Fixes: 98d81f0df70c ("nvme: use blk_mq_[un]quiesce_tagset")
> Signed-off-by: Mohamed Khalfella <mkhalfella@purestorage.com>

I think there is no better way to solve this in to nvme code ?

will it have any impact on existing users, if any, that are relying
on current mutex based implementation ?

BTW, thanks for reporting this and providing a patch.

> ---
>   block/blk-mq-sysfs.c   | 10 +++----
>   block/blk-mq.c         | 63 ++++++++++++++++++++++--------------------
>   include/linux/blk-mq.h |  4 +--
>   3 files changed, 40 insertions(+), 37 deletions(-)
>
> diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
> index 58ec293373c6..f474781654fb 100644
> --- a/block/blk-mq-sysfs.c
> +++ b/block/blk-mq-sysfs.c
> @@ -230,13 +230,13 @@ int blk_mq_sysfs_register(struct gendisk *disk)
>   
>   	kobject_uevent(q->mq_kobj, KOBJ_ADD);
>   
> -	mutex_lock(&q->tag_set->tag_list_lock);
> +	down_read(&q->tag_set->tag_list_rwsem);
>   	queue_for_each_hw_ctx(q, hctx, i) {
>   		ret = blk_mq_register_hctx(hctx);
>   		if (ret)
>   			goto out_unreg;
>   	}
> -	mutex_unlock(&q->tag_set->tag_list_lock);
> +	up_read(&q->tag_set->tag_list_rwsem);
>   	return 0;
>   

[...]

>   static void blk_mq_add_queue_tag_set(struct blk_mq_tag_set *set,
>   				     struct request_queue *q)
>   {
> -	mutex_lock(&set->tag_list_lock);
> +	down_write(&set->tag_list_rwsem);
> +	if (!list_is_singular(&set->tag_list)) {
> +		if (set->flags & BLK_MQ_F_TAG_QUEUE_SHARED)
> +			queue_set_hctx_shared(q, true);
> +		list_add_tail(&q->tag_set_list, &set->tag_list);
> +		up_write(&set->tag_list_rwsem);
> +		return;
> +	}
>   
> -	/*
> -	 * Check to see if we're transitioning to shared (from 1 to 2 queues).
> -	 */
> -	if (!list_empty(&set->tag_list) &&
> -	    !(set->flags & BLK_MQ_F_TAG_QUEUE_SHARED)) {
> -		set->flags |= BLK_MQ_F_TAG_QUEUE_SHARED;
> -		/* update existing queue */
> -		blk_mq_update_tag_set_shared(set, true);
> -	}
> -	if (set->flags & BLK_MQ_F_TAG_QUEUE_SHARED)
> -		queue_set_hctx_shared(q, true);
> +	/* Transitioning to shared. */
> +	set->flags |= BLK_MQ_F_TAG_QUEUE_SHARED;
>   	list_add_tail(&q->tag_set_list, &set->tag_list);
> -
> -	mutex_unlock(&set->tag_list_lock);
> +	downgrade_write(&set->tag_list_rwsem);

do we need a comment here what to expect since downgrade_write() is
not as common as mutex_unlock()|down_write() before merging the
patch ?

-ck