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Mon, 17 Nov 2025 21:31:06 -0500 X-MC-Unique: Od4xgRoSOTyjwaINJjq3RQ-1 X-Mimecast-MFC-AGG-ID: Od4xgRoSOTyjwaINJjq3RQ_1763433064 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 4D37918AB416; Tue, 18 Nov 2025 02:31:04 +0000 (UTC) Received: from fedora (unknown [10.72.116.204]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id B661B18002AC; Tue, 18 Nov 2025 02:30:57 +0000 (UTC) Date: Tue, 18 Nov 2025 10:30:52 +0800 From: Ming Lei To: Mohamed Khalfella Cc: Jens Axboe , Keith Busch , Sagi Grimberg , Chaitanya Kulkarni , Casey Chen , Vikas Manocha , Yuanyuan Zhong , Hannes Reinecke , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/1] nvme: Convert tag_list mutex to rwsemaphore to avoid deadlock Message-ID: References: <20251117202414.4071380-1-mkhalfella@purestorage.com> <20251117202414.4071380-2-mkhalfella@purestorage.com> <20251118021504.GC2197103-mkhalfella@purestorage.com> 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: <20251118021504.GC2197103-mkhalfella@purestorage.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 On Mon, Nov 17, 2025 at 06:15:04PM -0800, Mohamed Khalfella wrote: > On Tue 2025-11-18 10:00:19 +0800, Ming Lei wrote: > > On Mon, Nov 17, 2025 at 12:23:53PM -0800, Mohamed Khalfella wrote: > > > static void blk_mq_add_queue_tag_set(struct blk_mq_tag_set *set, > > > struct request_queue *q) > > > { > > > - mutex_lock(&set->tag_list_lock); > > > + struct request_queue *firstq; > > > + unsigned int memflags; > > > > > > - /* > > > - * 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); > > > - list_add_tail(&q->tag_set_list, &set->tag_list); > > > + 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; > > > + } > > > > > > - mutex_unlock(&set->tag_list_lock); > > > + /* Transitioning firstq and q to shared. */ > > > + set->flags |= BLK_MQ_F_TAG_QUEUE_SHARED; > > > + list_add_tail(&q->tag_set_list, &set->tag_list); > > > + downgrade_write(&set->tag_list_rwsem); > > > + queue_set_hctx_shared(q, true); > > > > queue_set_hctx_shared(q, true) should be moved into write critical area > > because this queue has been added to the list. > > > > I failed to see why that is the case. What can go wrong by running > queue_set_hctx_shared(q, true) after downgrade_write()? > > After the semaphore is downgraded we promise not to change the list > set->tag_list because now we have read-only access. Marking the "q" as > shared should be fine because it is new and we know there will be no > users of the queue yet (that is why we skipped freezing it). I think it is read/write lock's use practice. The protected data shouldn't be written any more when you downgrade to read lock. In this case, it may not make a difference, because it is one new queue and the other readers don't use the `shared` flag, but still better to do correct things from beginning and make code less fragile. Thanks, Ming