linux-kernel - Re: [PATCH 1/1] block: Use RCU in blk_mq_[un]quiesce_tagset() instead of set->tag_list

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Message-ID: <f24709c4-ba5e-4fb8-b20a-883d14185b2f@acm.org>
Date: Mon, 8 Dec 2025 11:22:33 -0800
From: Bart Van Assche <bvanassche@....org>
To: Keith Busch <kbusch@...nel.org>
Cc: Mohamed Khalfella <mkhalfella@...estorage.com>,
 Chaitanya Kulkarni <kch@...dia.com>, Christoph Hellwig <hch@....de>,
 Jens Axboe <axboe@...nel.dk>, Sagi Grimberg <sagi@...mberg.me>,
 Casey Chen <cachen@...estorage.com>, Yuanyuan Zhong
 <yzhong@...estorage.com>, Hannes Reinecke <hare@...e.de>,
 Ming Lei <ming.lei@...hat.com>, Waiman Long <llong@...hat.com>,
 Hillf Danton <hdanton@...a.com>, linux-nvme@...ts.infradead.org,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] block: Use RCU in blk_mq_[un]quiesce_tagset() instead
 of set->tag_list_lock

On 12/4/25 6:32 PM, Keith Busch wrote:
>   static void blk_mq_del_queue_tag_set(struct request_queue *q)
>   {
>   	struct blk_mq_tag_set *set = q->tag_set;
> +	struct request_queue *shared = NULL;
>   
>   	mutex_lock(&set->tag_list_lock);
>   	list_del(&q->tag_set_list);
> @@ -4302,15 +4305,25 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q)
>   		/* just transitioned to unshared */
>   		set->flags &= ~BLK_MQ_F_TAG_QUEUE_SHARED;
>   		/* update existing queue */
> -		blk_mq_update_tag_set_shared(set, false);
> +		shared = list_first_entry(&set->tag_list, struct request_queue,
> +					  tag_set_list);
> +		if (!blk_get_queue(shared))
> +			shared = NULL;
>   	}
>   	mutex_unlock(&set->tag_list_lock);
>   	INIT_LIST_HEAD(&q->tag_set_list);
> +
> +	if (shared) {
> +		queue_set_hctx_shared(shared);
> +		blk_put_queue(shared);
> +	}
>   }

Although harmless, with this approach the queue_set_hctx_shared() calls
by blk_mq_del_queue_tag_set() and blk_mq_add_queue_tag_set() can be
reordered. I like Mohamed's approach better because it results in code
that is easier to review and to reason about.

Thanks,

Bart.