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Fri, 14 Nov 2025 09:34:20 -0800 (PST) Date: Fri, 14 Nov 2025 09:34:19 -0800 From: Mohamed Khalfella To: Ming Lei 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: <20251114173419.GA2197103-mkhalfella@purestorage.com> References: <20251113202320.2530531-1-mkhalfella@purestorage.com> Precedence: bulk X-Mailing-List: linux-kernel@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: 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? 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?