Re: [PATCH V2] blk-mq: streamline handling of q->mq_ops->queue_rq result

[Ming Lei <ming.lei@redhat.com>]

On Wed, Jul 01, 2020 at 09:58:57PM +0800, Ming Lei wrote:
> Current handling of q->mq_ops->queue_rq result is a bit ugly:
> 
> - two branches which needs to 'continue' have to check if the
> dispatch local list is empty, otherwise one bad request may
> be retrieved via 'rq = list_first_entry(list, struct request, queuelist);'
> 
> - the branch of 'if (unlikely(ret != BLK_STS_OK))' isn't easy
> to follow, since it is actually one error branch.
> 
> Streamline this handling, so the code becomes more readable, meantime
> potential kernel oops can be avoided in case that the last request in
> local dispatch list is failed.
> 
> Fixes: fc17b6534eb8 ("blk-mq: switch ->queue_rq return value to blk_status_t")
> Cc: Christoph Hellwig <hch@infradead.org>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> V2:
> 	- change 'if else' to switch as suggested by Christoph
> 
>  block/blk-mq.c | 24 +++++++++++-------------
>  1 file changed, 11 insertions(+), 13 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 65e0846fd065..cc85775fc372 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1407,30 +1407,28 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list,
>  		if (nr_budgets)
>  			nr_budgets--;
>  		ret = q->mq_ops->queue_rq(hctx, &bd);
> -		if (ret == BLK_STS_RESOURCE || ret == BLK_STS_DEV_RESOURCE) {
> -			blk_mq_handle_dev_resource(rq, list);
> +		switch (ret) {
> +		case BLK_STS_OK:
> +			queued++;
>  			break;
> -		} else if (ret == BLK_STS_ZONE_RESOURCE) {
> +		case BLK_STS_RESOURCE:
> +		case BLK_STS_DEV_RESOURCE:
> +			blk_mq_handle_dev_resource(rq, list);
> +			goto out;
> +		case BLK_STS_ZONE_RESOURCE:
>  			/*
>  			 * Move the request to zone_list and keep going through
>  			 * the dispatch list to find more requests the drive can
>  			 * accept.
>  			 */
>  			blk_mq_handle_zone_resource(rq, &zone_list);
> -			if (list_empty(list))
> -				break;
> -			continue;
> -		}
> -
> -		if (unlikely(ret != BLK_STS_OK)) {
> +			break;
> +		default:
>  			errors++;
>  			blk_mq_end_request(rq, BLK_STS_IOERR);
> -			continue;
>  		}
> -
> -		queued++;
>  	} while (!list_empty(list));
> -
> +out:
>  	if (!list_empty(&zone_list))
>  		list_splice_tail_init(&zone_list, list);
>  
> -- 
> 2.25.2
> 

Hello,

Ping...

-- 
Ming