Re: [PATCH] block: Fix blk_mq_try_issue_directly()

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

On 4/8/19 10:36 AM, jianchao.wang wrote:
> 
> 
> On 4/8/19 10:07 AM, jianchao.wang wrote:
>> 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' ?
>>
> 
> The issue seems to be here,
> 
> blk_mq_try_issue_directly
> 
> 
>     if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q))) {
>         run_queue = false;
>         bypass = false;          //------> HERE !!!
>         goto out_unlock;
>     }
> 
> 
>     case BLK_STS_RESOURCE:
>         if (force) {
>             blk_mq_request_bypass_insert(rq, run_queue);
>             ret = bypass ? BLK_STS_OK : ret;
>         } else if (!bypass) {
>             blk_mq_sched_insert_request(rq, false,
>                             run_queue, false);
>         }
>         break;
> 
> Then the request will be inserted and blk_mq_try_issue_dreictly returns BLK_STS_RESOURCE.
> 
> 
> Could following patch fix the issue ?

Hi Laurence

Would you please test this patch to see whether the issue could be fixed ?

Thanks
Jianchao
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index a9c1816..a3394f2 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1813,7 +1813,7 @@ blk_status_t blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
>          */
>         if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q))) {
>                 run_queue = false;
> -               bypass = false;
> +               force = true;
>                 goto out_unlock;
>         }
> 
> 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:
>>>
>>
>