Re: [PATCH] block: Fix blk_mq_try_issue_directly()

["jianchao.wang" <jianchao.w.wang@oracle.com>]

Hi Bart

On 4/4/19 4:11 AM, Bart Van Assche wrote:
> If blk_mq_try_issue_directly() returns BLK_STS*_RESOURCE that means that
> the request has not been queued and that the caller should retry to submit
> the request. Both blk_mq_request_bypass_insert() and
> blk_mq_sched_insert_request() guarantee that a request will be processed.
> Hence return BLK_STS_OK if one of these functions is called. This patch
> avoids that blk_mq_dispatch_rq_list() crashes when using dm-mpath.

Sorry, I seem to miss the original mail list that reported this issue.
As your comment, it looks like that the request is handled again when
the blk_mq_try_issue_directly return BLK_STS*_RESOURCE, right ?

The usage of this helper interface is,
if care about the return value and want to handle the request yourself when
return BLK_STS*_RESOURCE, pass 'byass' as true.
otherwise, just pass 'bypass' as false, then blk_mq_try_issue_directly would
take over all of the work including requeue or complete the request.

if dm-mpath case, the driver should only invoke dm_dispatch_clone_request,
the 'bypass' parameter should only be true.
as the blk_mq_try_issue_directly,
it would return BLK_STS_OK when have to insert the request, otherwise,
it would do nothing but return BLK_STS*_RESOURCE.

Would you please show the cause that the dm-mpath driver invoke blk_mq_try_issue_direclty
with 'bypass == false' ?

Thanks
Jianchao



> 
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Hannes Reinecke <hare@suse.com>
> Cc: James Smart <james.smart@broadcom.com>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: Jianchao Wang <jianchao.w.wang@oracle.com>
> Cc: Keith Busch <keith.busch@intel.com>
> Cc: Dongli Zhang <dongli.zhang@oracle.com>
> Cc: Laurence Oberman <loberman@redhat.com>
> Tested-by: Laurence Oberman <loberman@redhat.com>
> Reviewed-by: Laurence Oberman <loberman@redhat.com>
> Reported-by: Laurence Oberman <loberman@redhat.com>
> Fixes: 7f556a44e61d ("blk-mq: refactor the code of issue request directly") # v5.0.
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
>  block/blk-mq.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 652d0c6d5945..b2c20dce8a30 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1859,16 +1859,11 @@ blk_status_t blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
>  	case BLK_STS_RESOURCE:
>  		if (force) {
>  			blk_mq_request_bypass_insert(rq, run_queue);
> -			/*
> -			 * We have to return BLK_STS_OK for the DM
> -			 * to avoid livelock. Otherwise, we return
> -			 * the real result to indicate whether the
> -			 * request is direct-issued successfully.
> -			 */
> -			ret = bypass ? BLK_STS_OK : ret;
> +			ret = BLK_STS_OK;
>  		} else if (!bypass) {
>  			blk_mq_sched_insert_request(rq, false,
>  						    run_queue, false);
> +			ret = BLK_STS_OK;
>  		}
>  		break;
>  	default:
>