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

Mohamed Khalfella
2025-12-04T19:03:03Z

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 <mkhalfella@purestorage.com>
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
>