Re: [PATCH v2] blk-mq: run queue no matter whether the request is the last request

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

On Tue, Jul 26, 2022 at 07:01:11PM +0800, Yufen Yu wrote:
> 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.

The story isn't related with scsi actually.

> 
> 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@redhat.com>
> Signed-off-by: Yufen Yu <yuyufen@huawei.com>

Reviewed-by: Ming Lei <ming.lei@redhat.com>

Thanks,
Ming