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

[Hannes Reinecke <hare@suse.de>]

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