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

[Bart Van Assche <bvanassche@acm.org>]

On 12/4/25 11:26 AM, Keith Busch wrote:
> On Thu, Dec 04, 2025 at 10:24:03AM -1000, Bart Van Assche wrote:>> Hence, the deadlock can be
>> solved by removing the blk_mq_quiesce_tagset() call from nvme_timeout()
>> and by failing I/O from inside nvme_timeout(). If nvme_timeout() fails
>> I/O and does not call blk_mq_quiesce_tagset() then the
>> blk_mq_freeze_queue_wait() call will finish instead of triggering a
>> deadlock. However, I do not know whether this proposal seems acceptable
>> to the NVMe maintainers.
> 
> You periodically make this suggestion, but there's never a reason
> offered to introduce yet another work queue for the driver to
> synchronize with at various points. The whole point of making blk-mq
> timeout handler in a work queue (it used to be a timer) was so that we
> could do blocking actions like this.
Hi Keith,

The blk_mq_quiesce_tagset() call from the NVMe timeout handler is
unfortunate because it triggers a deadlock with
blk_mq_update_tag_set_shared().

I proposed to modify the NVMe driver because I think that's a better
approach than introducing a new synchronize_rcu() call in the block
layer core.

However, there may be better approaches for fixing this in the NVMe
driver than what I proposed so far.

Thanks,

Bart.