On Tue, Jul 26, 2022 at 10:38:52AM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> 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.
>
> Fix the problem by also treat 'BLK_STS_RESOURCE' and
> 'BLK_STS_DEV_RESOURCE' as 'errors' in blk_mq_try_issue_list_directly(),
> so that 'commit_rqs' will be called when dispatch of last rq failed.
>
> Fixes: d666ba98f849 ("blk-mq: add mq_ops->commit_rqs()")
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
> block/blk-mq.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 70177ee74295..752b0fe4c128 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2680,6 +2680,7 @@ void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,
> list_del_init(&rq->queuelist);
> ret = blk_mq_request_issue_directly(rq, list_empty(list));
> if (ret != BLK_STS_OK) {
> + errors++;
OK, it is because that list becomes empty, but .queue_rq(last_rq_in_list)
returns BLK_STS_*RESOURCE, but scsi can't call ->queuecommand() for this
real last rq. Then blk_mq_try_issue_list_directly() doesn't call
->commit_rqs() too.
Here errors means that request not queued successfully, so this patch
looks fine.
Also I think blk_mq_dispatch_rq_list() needs similar handling too,
right?
thanks,
Ming
