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Thu, 04 Dec 2025 10:42:46 -0800 (PST) Date: Thu, 4 Dec 2025 10:42:43 -0800 From: Mohamed Khalfella To: Bart Van Assche Cc: Chaitanya Kulkarni , Christoph Hellwig , Jens Axboe , Keith Busch , Sagi Grimberg , Casey Chen , Yuanyuan Zhong , Hannes Reinecke , Ming Lei , Waiman Long , Hillf Danton , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/1] block: Use RCU in blk_mq_[un]quiesce_tagset() instead of set->tag_list_lock Message-ID: <20251204184243.GZ337106-mkhalfella@purestorage.com> References: <20251204181212.1484066-1-mkhalfella@purestorage.com> <20251204181212.1484066-2-mkhalfella@purestorage.com> <5450d3fa-3f00-40ae-ac95-1f08886de3b6@acm.org> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5450d3fa-3f00-40ae-ac95-1f08886de3b6@acm.org> 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.