Re: [PATCH v2] blk-mq: fix io hung due to missing commit_rqs

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

On Tue, Jul 26, 2022 at 11:35:19AM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> Currently, in virtio_scsi, if 'bd->last' is not set to true while
> dispatching request, such io will stay in driver's queue, and driver
> will wait for block layer to dispatch more rqs. However, if block
> layer failed to dispatch more rq, it should trigger commit_rqs to
> inform driver.
> 
> There is a problem in blk_mq_try_issue_list_directly() that commit_rqs
> won't be called:
> 
> // assume that queue_depth is set to 1, list contains two rq
> blk_mq_try_issue_list_directly
>  blk_mq_request_issue_directly
>  // dispatch first rq
>  // last is false
>   __blk_mq_try_issue_directly
>    blk_mq_get_dispatch_budget
>    // succeed to get first budget
>    __blk_mq_issue_directly
>     scsi_queue_rq
>      cmd->flags |= SCMD_LAST
>       virtscsi_queuecommand
>        kick = (sc->flags & SCMD_LAST) != 0
>        // kick is false, first rq won't issue to disk
>  queued++
> 
>  blk_mq_request_issue_directly
>  // dispatch second rq
>   __blk_mq_try_issue_directly
>    blk_mq_get_dispatch_budget
>    // failed to get second budget
>  ret == BLK_STS_RESOURCE
>   blk_mq_request_bypass_insert
>  // errors is still 0
> 
>  if (!list_empty(list) || errors && ...)
>   // won't pass, commit_rqs won't be called
> 
> In this situation, first rq relied on second rq to dispatch, while
> second rq relied on first rq to complete, thus they will both hung.
> And same problem exists in blk_mq_dispatch_rq_list()
> 
> Fix the problem by also treat 'BLK_STS_*RESOURCE' as 'errors' since
> it means that request is not queued successfully.
> 
> Fixes: d666ba98f849 ("blk-mq: add mq_ops->commit_rqs()")
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
> Changes in v2:
>  - suggested by Ming, handle blk_mq_dispatch_rq_list() as well.
>  - change title and modify commit message.
> 
>  block/blk-mq.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 70177ee74295..ee1e065fe63f 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1909,6 +1909,7 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list,
>  			fallthrough;
>  		case BLK_STS_DEV_RESOURCE:
>  			blk_mq_handle_dev_resource(rq, list);
> +			errors++;
>  			goto out;
>  		case BLK_STS_ZONE_RESOURCE:
>  			/*
> @@ -1918,6 +1919,7 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list,
>  			 */
>  			blk_mq_handle_zone_resource(rq, &zone_list);
>  			needs_resource = true;
> +			errors++;

But accounting error here may break return value of
blk_mq_dispatch_rq_list(), see:

/* Returns true if we did some work AND can potentially do more. */


thanks,
Ming