Re: [PATCH] block: Fix a deadlock related freezing zoned storage devices

[Jens Axboe <axboe@kernel.dk>]

On 5/22/25 11:14 AM, Bart Van Assche wrote:
> blk_mq_freeze_queue() never terminates if one or more bios are on the plug
> list and if the block device driver defines a .submit_bio() method.
> This is the case for device mapper drivers. The deadlock happens because
> blk_mq_freeze_queue() waits for q_usage_counter to drop to zero, because
> a queue reference is held by bios on the plug list and because the
> __bio_queue_enter() call in __submit_bio() waits for the queue to be
> unfrozen.
> 
> This patch fixes the following deadlock:
> 
> Workqueue: dm-51_zwplugs blk_zone_wplug_bio_work
> Call trace:
>  __schedule+0xb08/0x1160
>  schedule+0x48/0xc8
>  __bio_queue_enter+0xcc/0x1d0
>  __submit_bio+0x100/0x1b0
>  submit_bio_noacct_nocheck+0x230/0x49c
>  blk_zone_wplug_bio_work+0x168/0x250
>  process_one_work+0x26c/0x65c
>  worker_thread+0x33c/0x498
>  kthread+0x110/0x134
>  ret_from_fork+0x10/0x20
> 
> Call trace:
>  __switch_to+0x230/0x410
>  __schedule+0xb08/0x1160
>  schedule+0x48/0xc8
>  blk_mq_freeze_queue_wait+0x78/0xb8
>  blk_mq_freeze_queue+0x90/0xa4
>  queue_attr_store+0x7c/0xf0
>  sysfs_kf_write+0x98/0xc8
>  kernfs_fop_write_iter+0x12c/0x1d4
>  vfs_write+0x340/0x3ac
>  ksys_write+0x78/0xe8
> 
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Damien Le Moal <dlemoal@kernel.org>
> Cc: Yu Kuai <yukuai1@huaweicloud.com>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: stable@vger.kernel.org
> Fixes: dd291d77cc90 ("block: Introduce zone write plugging")
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
> 
> Changes compared to v1: fixed a race condition. Call bio_zone_write_plugging()
>   only before submitting the bio and not after it has been submitted.
> 
>  block/blk-core.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index b862c66018f2..713fb3865260 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -621,6 +621,13 @@ static inline blk_status_t blk_check_zone_append(struct request_queue *q,
>  	return BLK_STS_OK;
>  }
>  
> +/*
> + * Do not call bio_queue_enter() if the BIO_ZONE_WRITE_PLUGGING flag has been
> + * set because this causes blk_mq_freeze_queue() to deadlock if
> + * blk_zone_wplug_bio_work() submits a bio. Calling bio_queue_enter() for bios
> + * on the plug list is not necessary since a q_usage_counter reference is held
> + * while a bio is on the plug list.
> + */
>  static void __submit_bio(struct bio *bio)
>  {
>  	/* If plug is not used, add new plug here to cache nsecs time. */
> @@ -633,8 +640,12 @@ static void __submit_bio(struct bio *bio)
>  
>  	if (!bdev_test_flag(bio->bi_bdev, BD_HAS_SUBMIT_BIO)) {
>  		blk_mq_submit_bio(bio);
> -	} else if (likely(bio_queue_enter(bio) == 0)) {
> +	} else {
>  		struct gendisk *disk = bio->bi_bdev->bd_disk;
> +		bool zwp = bio_zone_write_plugging(bio);
> +
> +		if (unlikely(!zwp && bio_queue_enter(bio) != 0))
> +			goto finish_plug;
>  	
>  		if ((bio->bi_opf & REQ_POLLED) &&
>  		    !(disk->queue->limits.features & BLK_FEAT_POLL)) {
> @@ -643,9 +654,12 @@ static void __submit_bio(struct bio *bio)
>  		} else {
>  			disk->fops->submit_bio(bio);
>  		}
> -		blk_queue_exit(disk->queue);
> +
> +		if (!zwp)
> +			blk_queue_exit(disk->queue);
>  	}

This is pretty ugly, and I honestly absolutely hate how there's quite a
bit of zoned_whatever sprinkling throughout the core code. What's the
reason for not unplugging here, unaligned writes? Because you should
presumable have the exact same issues on non-zoned devices if they have
IO stuck in a plug (and doesn't get unplugged) while someone is waiting
on a freeze.

A somewhat similar case was solved for IOPOLL and queue entering. That
would be another thing to look at. Maybe a live enter could work if the
plug itself pins it?

-- 
Jens Axboe