Re: [PATCH v4 1/1] block: Use RCU in blk_mq_[un]quiesce_tagset() instead of set->tag_list_lock

[Ming Lei <ming.lei@redhat.com>]

On Fri, Dec 05, 2025 at 01:17:02PM -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+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 <mkhalfella@purestorage.com>
> Fixes: 98d81f0df70c ("nvme: use blk_mq_[un]quiesce_tagset")

Looks fine:

Reviewed-by: Ming Lei <ming.lei@redhat.com>


Thanks,
Ming