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

Hannes Reinecke
2025-12-09T07:30:33Z

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 <mkhalfella@purestorage.com>
> 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