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Thu, 04 Dec 2025 11:03:01 -0800 (PST) Date: Thu, 4 Dec 2025 11:02:58 -0800 From: Mohamed Khalfella To: Chaitanya Kulkarni , Christoph Hellwig , Jens Axboe , Keith Busch , Sagi Grimberg Cc: 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: <20251204190258.GA337106-mkhalfella@purestorage.com> References: <20251204181212.1484066-1-mkhalfella@purestorage.com> <20251204181212.1484066-2-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: <20251204181212.1484066-2-mkhalfella@purestorage.com> On Thu 2025-12-04 10:11:53 -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 > > The top stacktrace is showing nvme_timeout() called to handle nvme > command timeout. timeout handler is trying to disable the controller and > as a first step, it needs to blk_mq_quiesce_tagset() to tell blk-mq not > to call queue callback handlers. The thread is stuck waiting for > set->tag_list_lock as it tires to walk the queues in set->tag_list. > > The lock is held by the second thread in the bottom stack which is > waiting for one of queues to be frozen. The queue usage counter will > drop to zero after nvme_timeout() finishes, and this will not happen > because the thread will wait for this mutex forever. > > Given that [un]quescing queue is an operation that does not need to > sleep, update blk_mq_[un]quiesce_tagset() to use RCU instead of taking > set->tag_list_lock. Also update blk_mq_{add,del}_queue_tag_set() to use > RCU safe list operations. This should help avoid deadlock seen above. > > Signed-off-by: Mohamed Khalfella Fixes: 98d81f0df70c ("nvme: use blk_mq_[un]quiesce_tagset") Oops, this should be v3 and I also missed Fixes: tag above. > --- > block/blk-mq.c | 18 ++++++++++-------- > 1 file changed, 10 insertions(+), 8 deletions(-) > > diff --git a/block/blk-mq.c b/block/blk-mq.c > index d626d32f6e57..ceb176ac154b 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -335,12 +335,12 @@ void blk_mq_quiesce_tagset(struct blk_mq_tag_set *set) > { > struct request_queue *q; > > - mutex_lock(&set->tag_list_lock); > - list_for_each_entry(q, &set->tag_list, tag_set_list) { > + rcu_read_lock(); > + list_for_each_entry_rcu(q, &set->tag_list, tag_set_list) { > if (!blk_queue_skip_tagset_quiesce(q)) > blk_mq_quiesce_queue_nowait(q); > } > - mutex_unlock(&set->tag_list_lock); > + rcu_read_unlock(); > > blk_mq_wait_quiesce_done(set); > } > @@ -350,12 +350,12 @@ void blk_mq_unquiesce_tagset(struct blk_mq_tag_set *set) > { > struct request_queue *q; > > - mutex_lock(&set->tag_list_lock); > - list_for_each_entry(q, &set->tag_list, tag_set_list) { > + rcu_read_lock(); > + list_for_each_entry_rcu(q, &set->tag_list, tag_set_list) { > if (!blk_queue_skip_tagset_quiesce(q)) > blk_mq_unquiesce_queue(q); > } > - mutex_unlock(&set->tag_list_lock); > + rcu_read_unlock(); > } > EXPORT_SYMBOL_GPL(blk_mq_unquiesce_tagset); > > @@ -4294,7 +4294,7 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q) > struct blk_mq_tag_set *set = q->tag_set; > > mutex_lock(&set->tag_list_lock); > - list_del(&q->tag_set_list); > + list_del_rcu(&q->tag_set_list); > if (list_is_singular(&set->tag_list)) { > /* just transitioned to unshared */ > set->flags &= ~BLK_MQ_F_TAG_QUEUE_SHARED; > @@ -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); > } > > @@ -4321,7 +4323,7 @@ static void blk_mq_add_queue_tag_set(struct blk_mq_tag_set *set, > } > 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); > + list_add_tail_rcu(&q->tag_set_list, &set->tag_list); > > mutex_unlock(&set->tag_list_lock); > } > -- > 2.51.2 >