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: <aTMgS0njg-R-T0pT@kbusch-mbp>
Date: Fri, 5 Dec 2025 11:11:23 -0700
From: Keith Busch <kbusch@...nel.org>
To: Mohamed Khalfella <mkhalfella@...estorage.com>
Cc: Bart Van Assche <bvanassche@....org>,
	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 Fri, Dec 05, 2025 at 08:39:26AM -0800, Mohamed Khalfella wrote:
> Why sychronize_rcu() is intolerable in this blk_mq_del_queue_tag_set()?
> This code is not performance sensitive, right?

synchronize_rcu() gets expensive on servers with many CPUs. While this
is not a fast path, I think adding it here would still be harmful, if
only just to testing when devices are frequently created and deleted.

> Looking at the code again, I _think_ synchronize_rcu() along with 
> INIT_LIST_HEAD(&q->tag_set_list) can be deleted. I do not see usecase
> where "q" is re-added to a tagset after it is deleted from one.
> Also, "q" is freed in blk_free_queue() after end of RCU grace period.

I think you could skip the synchronize since the queue's memory free is
done in blk_free_queue_rcu from a call_rcu() callback.