On Wed, Aug 3, 2022 at 10:22 AM Yufen Yu <yuyufen@xxxxxxxxxxxxxxx> wrote:
>
> From: Yufen Yu <yuyufen@xxxxxxxxxx>
>
> We do test on a virtio scsi device (/dev/sda) and the default mq
> scheduler is 'none'. We found a IO hung as following:
>
> blk_finish_plug
> blk_mq_plug_issue_direct
> scsi_mq_get_budget
> //get budget_token fail and sdev->restarts=1
>
> scsi_end_request
> scsi_run_queue_async
> //sdev->restart=0 and run queue
>
> blk_mq_request_bypass_insert
> //add request to hctx->dispatch list
>
> //continue to dispath plug list
> blk_mq_dispatch_plug_list
> blk_mq_try_issue_list_directly
> //success issue all requests from plug list
>
> After .get_budget fail, scsi_mq_get_budget will increase 'restarts'.
> Normally, it will run hw queue when io complete and set 'restarts'
> as 0. But if we run queue before adding request to the dispatch list
> and blk_mq_dispatch_plug_list also success issue all requests, then
> on one will run queue, and the request will be stall in the dispatch
> list and cannot complete forever.
>
> It is wrong to use last request of plug list to decide if run queue is
> needed since all the remained requests in plug list may be from other
> hctxs. To fix the bug, pass run_queue as true always to
> blk_mq_request_bypass_insert().
>
> Fix-suggested-by: Ming Lei <ming.lei@xxxxxxxxxx>
> Signed-off-by: Yufen Yu <yuyufen@xxxxxxxxxx>
> Reviewed-by: Ming Lei <ming.lei@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 93d9d60980fb..1eb13d57a946 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2568,7 +2568,7 @@ static void blk_mq_plug_issue_direct(struct blk_plug *plug, bool from_schedule)
> break;
> case BLK_STS_RESOURCE:
> case BLK_STS_DEV_RESOURCE:
> - blk_mq_request_bypass_insert(rq, false, last);
> + blk_mq_request_bypass_insert(rq, false, true);
> blk_mq_commit_rqs(hctx, &queued, from_schedule);
> return;
> default:
Fixes: dc5fc361d891 ("block: attempt direct issue of plug list")
Thanks,
Ming
