Message-ID: <aRvjQ6QluONcObXD@fedora>
Date: Tue, 18 Nov 2025 11:08:51 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Waiman Long <llong@...hat.com>
Cc: Hillf Danton <hdanton@...a.com>,
	Mohamed Khalfella <mkhalfella@...estorage.com>,
	Jens Axboe <axboe@...nel.dk>, linux-nvme@...ts.infradead.org,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] nvme: Convert tag_list mutex to rwsemaphore to
 avoid deadlock

On Mon, Nov 17, 2025 at 09:24:21PM -0500, Waiman Long wrote:
> On 11/17/25 8:34 PM, Hillf Danton wrote:
> > On Mon, 17 Nov 2025 12:23:53 -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.
> > > 
> > > Convert set->tag_list_lock mutex to set->tag_list_rwsem rwsemaphore to
> > > avoid the deadlock. Update blk_mq_[un]quiesce_tagset() to take the
> > > semaphore for read since this is enough to guarantee no queues will be
> > > added or removed. Update blk_mq_{add,del}_queue_tag_set() to take the
> > > semaphore for write while updating set->tag_list and downgrade it to
> > > read while freezing the queues. It should be safe to update set->flags
> > > and hctx->flags while holding the semaphore for read since the queues
> > > are already frozen.
> > > 
> > > Fixes: 98d81f0df70c ("nvme: use blk_mq_[un]quiesce_tagset")
> > > Signed-off-by: Mohamed Khalfella <mkhalfella@...estorage.com>
> > > ---
> > >   block/blk-mq-sysfs.c   | 10 ++---
> > >   block/blk-mq.c         | 95 +++++++++++++++++++++++-------------------
> > >   include/linux/blk-mq.h |  4 +-
> > >   3 files changed, 58 insertions(+), 51 deletions(-)
> > > 
> > > diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
> > > index 58ec293373c6..f474781654fb 100644
> > > --- a/block/blk-mq-sysfs.c
> > > +++ b/block/blk-mq-sysfs.c
> > > @@ -230,13 +230,13 @@ int blk_mq_sysfs_register(struct gendisk *disk)
> > >   	kobject_uevent(q->mq_kobj, KOBJ_ADD);
> > > -	mutex_lock(&q->tag_set->tag_list_lock);
> > > +	down_read(&q->tag_set->tag_list_rwsem);
> > >   	queue_for_each_hw_ctx(q, hctx, i) {
> > >   		ret = blk_mq_register_hctx(hctx);
> > >   		if (ret)
> > >   			goto out_unreg;
> > >   	}
> > > -	mutex_unlock(&q->tag_set->tag_list_lock);
> > > +	up_read(&q->tag_set->tag_list_rwsem);
> > >   	return 0;
> > >   out_unreg:
> > > @@ -244,7 +244,7 @@ int blk_mq_sysfs_register(struct gendisk *disk)
> > >   		if (j < i)
> > >   			blk_mq_unregister_hctx(hctx);
> > >   	}
> > > -	mutex_unlock(&q->tag_set->tag_list_lock);
> > > +	up_read(&q->tag_set->tag_list_rwsem);
> > >   	kobject_uevent(q->mq_kobj, KOBJ_REMOVE);
> > >   	kobject_del(q->mq_kobj);
> > > @@ -257,10 +257,10 @@ void blk_mq_sysfs_unregister(struct gendisk *disk)
> > >   	struct blk_mq_hw_ctx *hctx;
> > >   	unsigned long i;
> > > -	mutex_lock(&q->tag_set->tag_list_lock);
> > > +	down_read(&q->tag_set->tag_list_rwsem);
> > >   	queue_for_each_hw_ctx(q, hctx, i)
> > >   		blk_mq_unregister_hctx(hctx);
> > > -	mutex_unlock(&q->tag_set->tag_list_lock);
> > > +	up_read(&q->tag_set->tag_list_rwsem);
> > >   	kobject_uevent(q->mq_kobj, KOBJ_REMOVE);
> > >   	kobject_del(q->mq_kobj);
> > > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > > index d626d32f6e57..9211d32ce820 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);
> > > +	down_read(&set->tag_list_rwsem);
> > >   	list_for_each_entry(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);
> > > +	up_read(&set->tag_list_rwsem);
> > >   	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);
> > > +	down_read(&set->tag_list_rwsem);
> > >   	list_for_each_entry(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);
> > > +	up_read(&set->tag_list_rwsem);
> > >   }
> > >   EXPORT_SYMBOL_GPL(blk_mq_unquiesce_tagset);
> > > @@ -4274,56 +4274,63 @@ static void queue_set_hctx_shared(struct request_queue *q, bool shared)
> > >   	}
> > >   }
> > > -static void blk_mq_update_tag_set_shared(struct blk_mq_tag_set *set,
> > > -					 bool shared)
> > > -{
> > > -	struct request_queue *q;
> > > -	unsigned int memflags;
> > > -
> > > -	lockdep_assert_held(&set->tag_list_lock);
> > > -
> > > -	list_for_each_entry(q, &set->tag_list, tag_set_list) {
> > > -		memflags = blk_mq_freeze_queue(q);
> > > -		queue_set_hctx_shared(q, shared);
> > > -		blk_mq_unfreeze_queue(q, memflags);
> > > -	}
> > > -}
> > > -
> > >   static void blk_mq_del_queue_tag_set(struct request_queue *q)
> > >   {
> > >   	struct blk_mq_tag_set *set = q->tag_set;
> > > +	struct request_queue *firstq;
> > > +	unsigned int memflags;
> > > -	mutex_lock(&set->tag_list_lock);
> > > +	down_write(&set->tag_list_rwsem);
> > >   	list_del(&q->tag_set_list);
> > > -	if (list_is_singular(&set->tag_list)) {
> > > -		/* just transitioned to unshared */
> > > -		set->flags &= ~BLK_MQ_F_TAG_QUEUE_SHARED;
> > > -		/* update existing queue */
> > > -		blk_mq_update_tag_set_shared(set, false);
> > > +	if (!list_is_singular(&set->tag_list)) {
> > > +		up_write(&set->tag_list_rwsem);
> > > +		goto out;
> > >   	}
> > > -	mutex_unlock(&set->tag_list_lock);
> > > +
> > > +	/*
> > > +	 * Transitioning the remaining firstq to unshared.
> > > +	 * Also, downgrade the semaphore to avoid deadlock
> > > +	 * with blk_mq_quiesce_tagset() while waiting for
> > > +	 * firstq to be frozen.
> > > +	 */
> > > +	set->flags &= ~BLK_MQ_F_TAG_QUEUE_SHARED;
> > > +	downgrade_write(&set->tag_list_rwsem);
> > If the first lock waiter is for write, it could ruin your downgrade trick.

If the 1st waiter is for WEITE, rwsem_mark_wake() simply returns and grants
read lock to this caller, meantime wakes up nothing.

That is exactly what this use case expects, so can you explain in detail why
`it could ruin your downgrade trick`?

> 
> That is true. The downgrade will wake up all the waiting readers at the
> front of the wait queue, but if there is one or more writers in the mix. The
> wakeup will stop when the first writer is hit and all the readers after that
> will not be woken up.

So waiters for WRITE won't be waken up by downgrade_write() if I understand correctly,
and rwsem_downgrade_wake() documents this behavior too.

> 
> We can theoretically provide a downgrade variant that wakes up all the
> readers if it is a useful feature.

The following up_read() in this code block will wake up the waiter for
WRITE, which finally wakes up other waiters for READ, then I am confused
what is the problem with this usage?


Thanks,
Ming

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