Re: [PATCH v2 11/12] block: mq-deadline: Fix a race condition related to zoned writes

[Damien Le Moal <dlemoal@kernel.org>]

On 4/8/23 08:58, Bart Van Assche wrote:
> Let deadline_next_request() only consider the first zoned write per
> zone. This patch fixes a race condition between deadline_next_request()
> and completion of zoned writes.
> 
> Cc: Damien Le Moal <damien.lemoal@opensource.wdc.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: Mike Snitzer <snitzer@kernel.org>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
>  block/mq-deadline.c    | 24 +++++++++++++++++++++---
>  include/linux/blk-mq.h |  5 +++++
>  2 files changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
> index 8c2bc9fdcf8c..d49e20d3011d 100644
> --- a/block/mq-deadline.c
> +++ b/block/mq-deadline.c
> @@ -389,12 +389,30 @@ deadline_next_request(struct deadline_data *dd, struct dd_per_prio *per_prio,
>  	 */
>  	spin_lock_irqsave(&dd->zone_lock, flags);
>  	while (rq) {
> +		unsigned int zno = blk_rq_zone_no(rq);
> +
>  		if (blk_req_can_dispatch_to_zone(rq))
>  			break;
> -		if (blk_queue_nonrot(q))
> -			rq = deadline_latter_request(rq);
> -		else
> +
> +		WARN_ON_ONCE(!blk_queue_is_zoned(q));

I do not think this WARN is useful as blk_req_can_dispatch_to_zone() will always
return true for regular block devices.

> +
> +		if (!blk_queue_nonrot(q)) {
>  			rq = deadline_skip_seq_writes(dd, rq);
> +			if (!rq)
> +				break;
> +			rq = deadline_earlier_request(rq);
> +			if (WARN_ON_ONCE(!rq))
> +				break;

I do not understand why this is needed.

> +		}
> +
> +		/*
> +		 * Skip all other write requests for the zone with zone number
> +		 * 'zno'. This prevents that this function selects a zoned write
> +		 * that is not the first write for a given zone.
> +		 */
> +		while ((rq = deadline_latter_request(rq)) &&
> +		       blk_rq_zone_no(rq) == zno)
> +			;
>  	}
>  	spin_unlock_irqrestore(&dd->zone_lock, flags);
>  
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index e62feb17af96..515dfd04d736 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -1193,6 +1193,11 @@ static inline bool blk_req_can_dispatch_to_zone(struct request *rq)
>  	return !blk_req_zone_is_write_locked(rq);
>  }
>  #else /* CONFIG_BLK_DEV_ZONED */
> +static inline unsigned int blk_rq_zone_no(struct request *rq)
> +{
> +	return 0;
> +}
> +
>  static inline bool blk_req_needs_zone_write_lock(struct request *rq)
>  {
>  	return false;