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: <20251204184243.GZ337106-mkhalfella@purestorage.com>
Date: Thu, 4 Dec 2025 10:42:43 -0800
From: Mohamed Khalfella <mkhalfella@...estorage.com>
To: Bart Van Assche <bvanassche@....org>
Cc: Chaitanya Kulkarni <kch@...dia.com>, Christoph Hellwig <hch@....de>,
	Jens Axboe <axboe@...nel.dk>, Keith Busch <kbusch@...nel.org>,
	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 Thu 2025-12-04 08:22:23 -1000, Bart Van Assche wrote:
> On 12/4/25 8:11 AM, Mohamed Khalfella wrote:
> > @@ -4302,6 +4302,8 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q)
> >   		blk_mq_update_tag_set_shared(set, false);
> >   	}
> >   	mutex_unlock(&set->tag_list_lock);
> > +
> > +	synchronize_rcu();
> >   	INIT_LIST_HEAD(&q->tag_set_list);
> >   }
> Yikes. This change slows down all blk_mq_del_queue_tag_set() callers.

synchronize_rcu() is necessary before re-initializing q->tag_set_list
because of list_for_each_entry_rcu() in blk_mq_[un]quiesce_tagset().

Is blk_mq_del_queue_tag_set() performance sensitive such that it can not
take synchronize_rcu()? It is not in IO codepath, right?

> Please fix the reported deadlock by modifying the NVMe code instead of
> slowing down the block layer.

I can not think of an easy way to do that. Suggestions are welcomed.

> 
> Thanks,
> 
> Bart.