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Tue, 9 Dec 2025 07:30:26 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id QSKwIxLQN2nuSgAAD6G6ig (envelope-from ); Tue, 09 Dec 2025 07:30:26 +0000 Message-ID: Date: Tue, 9 Dec 2025 08:30:23 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 1/1] block: Use RCU in blk_mq_[un]quiesce_tagset() instead of set->tag_list_lock To: Mohamed Khalfella , Chaitanya Kulkarni , Christoph Hellwig , Jens Axboe , Keith Busch , Sagi Grimberg Cc: Casey Chen , Yuanyuan Zhong , Ming Lei , Waiman Long , Hillf Danton , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <20251205211738.1872244-1-mkhalfella@purestorage.com> <20251205211738.1872244-2-mkhalfella@purestorage.com> Content-Language: en-US From: Hannes Reinecke In-Reply-To: <20251205211738.1872244-2-mkhalfella@purestorage.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Flag: NO X-Spam-Score: -4.30 X-Spam-Level: X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-0.980]; MIME_GOOD(-0.10)[text/plain]; ARC_NA(0.00)[]; RCPT_COUNT_TWELVE(0.00)[14]; MIME_TRACE(0.00)[0:+]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; TO_MATCH_ENVRCPT_ALL(0.00)[]; FUZZY_RATELIMITED(0.00)[rspamd.com]; FREEMAIL_ENVRCPT(0.00)[sina.com]; RCVD_TLS_ALL(0.00)[]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[purestorage.com,redhat.com,sina.com,lists.infradead.org,vger.kernel.org]; RCVD_COUNT_TWO(0.00)[2]; RCVD_VIA_SMTP_AUTH(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:email,suse.de:mid,imap1.dmz-prg2.suse.org:helo] On 12/5/25 22:17, 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+0x47c/0xbb0 > ? timerqueue_add+0x66/0xb0 > schedule+0x1c/0xa0 > schedule_preempt_disabled+0xa/0x10 > __mutex_lock.constprop.0+0x271/0x600 > blk_mq_quiesce_tagset+0x25/0xc0 > nvme_dev_disable+0x9c/0x250 > nvme_timeout+0x1fc/0x520 > blk_mq_handle_expired+0x5c/0x90 > bt_iter+0x7e/0x90 > blk_mq_queue_tag_busy_iter+0x27e/0x550 > ? __blk_mq_complete_request_remote+0x10/0x10 > ? __blk_mq_complete_request_remote+0x10/0x10 > ? __call_rcu_common.constprop.0+0x1c0/0x210 > blk_mq_timeout_work+0x12d/0x170 > process_one_work+0x12e/0x2d0 > worker_thread+0x288/0x3a0 > ? rescuer_thread+0x480/0x480 > kthread+0xb8/0xe0 > ? kthread_park+0x80/0x80 > ret_from_fork+0x2d/0x50 > ? kthread_park+0x80/0x80 > ret_from_fork_asm+0x11/0x20 > > __schedule+0x47c/0xbb0 > ? xas_find+0x161/0x1a0 > schedule+0x1c/0xa0 > blk_mq_freeze_queue_wait+0x3d/0x70 > ? destroy_sched_domains_rcu+0x30/0x30 > blk_mq_update_tag_set_shared+0x44/0x80 > blk_mq_exit_queue+0x141/0x150 > del_gendisk+0x25a/0x2d0 > nvme_ns_remove+0xc9/0x170 > nvme_remove_namespaces+0xc7/0x100 > nvme_remove+0x62/0x150 > pci_device_remove+0x23/0x60 > device_release_driver_internal+0x159/0x200 > unbind_store+0x99/0xa0 > kernfs_fop_write_iter+0x112/0x1e0 > vfs_write+0x2b1/0x3d0 > ksys_write+0x4e/0xb0 > do_syscall_64+0x5b/0x160 > entry_SYSCALL_64_after_hwframe+0x4b/0x53 > > 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 tries 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]quiescing 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, update blk_mq_{add,del}_queue_tag_set() to use RCU > safe list operations. Also, delete INIT_LIST_HEAD(&q->tag_set_list) > in blk_mq_del_queue_tag_set() because we can not re-initialize it while > the list is being traversed under RCU. The deleted queue will not be > added/deleted to/from a tagset and it will be freed in blk_free_queue() > after the end of RCU grace period. > > Signed-off-by: Mohamed Khalfella > Fixes: 98d81f0df70c ("nvme: use blk_mq_[un]quiesce_tagset") > --- > block/blk-mq.c | 17 ++++++++--------- > 1 file changed, 8 insertions(+), 9 deletions(-) > > diff --git a/block/blk-mq.c b/block/blk-mq.c > index d626d32f6e57..05db3d20783f 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; > @@ -4302,7 +4302,6 @@ 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); > - INIT_LIST_HEAD(&q->tag_set_list); > } > I'm ever so sceptical whether we can remove the INIT_LIST_HEAD() here. If we can it was pointless to begin with, but I somehow doubt that. Do you have a rationale for that (except from the fact that you are moving to RCU, and hence the 'q' pointer might not be valid then). Cheers, Hannes -- Dr. Hannes Reinecke Kernel Storage Architect hare@suse.de +49 911 74053 688 SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich