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Sun, 16 Nov 2025 21:13:16 -0500 X-MC-Unique: QjYhJkZoMDmlt1lRuliI9w-1 X-Mimecast-MFC-AGG-ID: QjYhJkZoMDmlt1lRuliI9w_1763345594 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (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-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 6552019560A1; Mon, 17 Nov 2025 02:13:13 +0000 (UTC) Received: from fedora (unknown [10.72.116.55]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 8573719560B0; Mon, 17 Nov 2025 02:13:05 +0000 (UTC) Date: Mon, 17 Nov 2025 10:13:00 +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] nvme: Convert tag_list mutex to rwsemaphore to avoid deadlock Message-ID: References: <20251113202320.2530531-1-mkhalfella@purestorage.com> <20251114173419.GA2197103-mkhalfella@purestorage.com> <20251116164606.GA2376676-mkhalfella@purestorage.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251116164606.GA2376676-mkhalfella@purestorage.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20251116_181319_812359_214A0ED1 X-CRM114-Status: GOOD ( 41.89 ) X-BeenThere: linux-nvme@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "Linux-nvme" Errors-To: linux-nvme-bounces+linux-nvme=archiver.kernel.org@lists.infradead.org On Sun, Nov 16, 2025 at 08:46:06AM -0800, Mohamed Khalfella wrote: > On Sun 2025-11-16 23:15:38 +0800, Ming Lei wrote: > > On Fri, Nov 14, 2025 at 09:34:19AM -0800, Mohamed Khalfella wrote: > > > On Fri 2025-11-14 19:41:14 +0800, Ming Lei wrote: > > > > On Thu, Nov 13, 2025 at 12:23:20PM -0800, Mohamed Khalfella wrote: > > > > > blk_mq_{add,del}_queue_tag_set() functions add and remove queues from > > > > > tagset, the functions make sure that tagset and queues are marked as > > > > > shared when two or more queues are attached to the same tagset. > > > > > Initially a tagset starts as unshared and when the number of added > > > > > queues reaches two, blk_mq_add_queue_tag_set() marks it as shared along > > > > > with all the queues attached to it. When the number of attached queues > > > > > drops to 1 blk_mq_del_queue_tag_set() need to mark both the tagset and > > > > > the remaining queues as unshared. > > > > > > > > > > Both functions need to freeze current queues in tagset before setting on > > > > > unsetting BLK_MQ_F_TAG_QUEUE_SHARED flag. While doing so, both functions > > > > > hold set->tag_list_lock mutex, which makes sense as we do not want > > > > > queues to be added or deleted in the process. This used to work fine > > > > > until commit 98d81f0df70c ("nvme: use blk_mq_[un]quiesce_tagset") > > > > > made the nvme driver quiesce tagset instead of quiscing individual > > > > > queues. blk_mq_quiesce_tagset() does the job and quiesce the queues in > > > > > set->tag_list while holding set->tag_list_lock also. > > > > > > > > > > This results in deadlock between two threads with these stacktraces: > > > > > > > > > > __schedule+0x48e/0xed0 > > > > > schedule+0x5a/0xc0 > > > > > schedule_preempt_disabled+0x11/0x20 > > > > > __mutex_lock.constprop.0+0x3cc/0x760 > > > > > blk_mq_quiesce_tagset+0x26/0xd0 > > > > > nvme_dev_disable_locked+0x77/0x280 [nvme] > > > > > nvme_timeout+0x268/0x320 [nvme] > > > > > blk_mq_handle_expired+0x5d/0x90 > > > > > bt_iter+0x7e/0x90 > > > > > blk_mq_queue_tag_busy_iter+0x2b2/0x590 > > > > > ? __blk_mq_complete_request_remote+0x10/0x10 > > > > > ? __blk_mq_complete_request_remote+0x10/0x10 > > > > > blk_mq_timeout_work+0x15b/0x1a0 > > > > > process_one_work+0x133/0x2f0 > > > > > ? mod_delayed_work_on+0x90/0x90 > > > > > worker_thread+0x2ec/0x400 > > > > > ? mod_delayed_work_on+0x90/0x90 > > > > > kthread+0xe2/0x110 > > > > > ? kthread_complete_and_exit+0x20/0x20 > > > > > ret_from_fork+0x2d/0x50 > > > > > ? kthread_complete_and_exit+0x20/0x20 > > > > > ret_from_fork_asm+0x11/0x20 > > > > > > > > > > __schedule+0x48e/0xed0 > > > > > schedule+0x5a/0xc0 > > > > > blk_mq_freeze_queue_wait+0x62/0x90 > > > > > ? destroy_sched_domains_rcu+0x30/0x30 > > > > > blk_mq_exit_queue+0x151/0x180 > > > > > disk_release+0xe3/0xf0 > > > > > device_release+0x31/0x90 > > > > > kobject_put+0x6d/0x180 > > > > > nvme_scan_ns+0x858/0xc90 [nvme_core] > > > > > ? nvme_scan_work+0x281/0x560 [nvme_core] > > > > > nvme_scan_work+0x281/0x560 [nvme_core] > > > > > process_one_work+0x133/0x2f0 > > > > > ? mod_delayed_work_on+0x90/0x90 > > > > > worker_thread+0x2ec/0x400 > > > > > ? mod_delayed_work_on+0x90/0x90 > > > > > kthread+0xe2/0x110 > > > > > ? kthread_complete_and_exit+0x20/0x20 > > > > > ret_from_fork+0x2d/0x50 > > > > > ? kthread_complete_and_exit+0x20/0x20 > > > > > ret_from_fork_asm+0x11/0x20 > > > > > > > > It is one AB-BA deadlock, lockdep should have complained it, but nvme doesn't > > > > support owned freeze queue. > > > > > > > > Maybe the following change can avoid it? > > > > > > > > diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c > > > > index c916176bd9f0..9967c4a7e72d 100644 > > > > --- a/drivers/nvme/host/pci.c > > > > +++ b/drivers/nvme/host/pci.c > > > > @@ -3004,6 +3004,7 @@ static void nvme_dev_disable(struct nvme_dev *dev, bool shutdown) > > > > bool dead; > > > > > > > > mutex_lock(&dev->shutdown_lock); > > > > + nvme_quiesce_io_queues(&dev->ctrl); > > > > dead = nvme_pci_ctrl_is_dead(dev); > > > > if (state == NVME_CTRL_LIVE || state == NVME_CTRL_RESETTING) { > > > > if (pci_is_enabled(pdev)) > > > > @@ -3016,8 +3017,6 @@ static void nvme_dev_disable(struct nvme_dev *dev, bool shutdown) > > > > nvme_wait_freeze_timeout(&dev->ctrl, NVME_IO_TIMEOUT); > > > > } > > > > > > > > - nvme_quiesce_io_queues(&dev->ctrl); > > > > - > > > > if (!dead && dev->ctrl.queue_count > 0) { > > > > nvme_delete_io_queues(dev); > > > > nvme_disable_ctrl(&dev->ctrl, shutdown); > > > > > > > > > > > > > > Interesting. Can you elaborate more on why this diff can help us avoid > > > the problem? > > > > In nvme_dev_disable(), NS queues are frozen first, then call > > nvme_quiesce_io_queues(). > > > > queue freeze can be thought as one lock, so q->freeze_lock -> tag_set->tag_list_lock > > in timeout code path. > > > > However, in blk_mq_exit_queue(), the lock order becomes > > tag_set->tag_list_lock -> q->freeze_lock. > > > > That is why I call it AB-BA lock. > > > > However, that looks not the reason in your deadlock, so my patch shouldn't > > work here, in which nvme_dev_disable() doesn't provide forward-progress > > because of ->tag_list_lock. > > > > > > > > If the thread doing nvme_scan_work() is waiting for queue to be frozen > > > while holding set->tag_list_lock, then nvme_quiesce_io_queues() moved up > > > will cause the deadlock, no? > > > > __schedule+0x48e/0xed0 > > schedule+0x5a/0xc0 > > blk_mq_freeze_queue_wait+0x62/0x90 > > ? destroy_sched_domains_rcu+0x30/0x30 > > blk_mq_exit_queue+0x151/0x180 > > disk_release+0xe3/0xf0 > > device_release+0x31/0x90 > > kobject_put+0x6d/0x180 > > > > Here blk_mq_exit_queue() is called from disk release, and the disk is not > > added actually, so question is why blk_mq_freeze_queue_wait() doesn't > > return? Who holds queue usage counter here? Can you investigate a bit > > and figure out the reason? > > The nvme controller has two namespaces. n1 was created with q1 added to > the tagset. n2 was also created with q2 added to the tagset. Now the > tagset is shared because it has two queues on it. Before the disk is > added n2 was found to be duplicate to n1. That means the disk should be > released, as noted above, and q2 should be removed from the tagset. > Because n1 block device is ready to receive IO it is possible for q1 > usage counter to be greater than zero. > > Back to q2 removal from the tagset. This requires q1 to be frozen before > it can be marked as unshared. blk_mq_freeze_queue_wait() was called for > q1 and it does not return because there is a request issued on the queue. > nvme_timeout() was called for that request in q1. Yeah, it is actually the other request queues frozen from blk_mq_exit_queue(). Then your approach looks good, will run a close view. Thanks, Ming