Re: [PATCH] blk-mq: put the driver tag of nxt rq before first one is requeued

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

On 09/13/2017 10:23 AM, Jens Axboe wrote:
> On 09/12/2017 07:39 PM, jianchao.wang wrote:
>>
>>
>> On 09/13/2017 09:24 AM, Ming Lei wrote:
>>> On Wed, Sep 13, 2017 at 09:01:25AM +0800, jianchao.wang wrote:
>>>> Hi ming
>>>>
>>>> On 09/12/2017 06:23 PM, Ming Lei wrote:
>>>>>> @@ -1029,14 +1029,20 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
>>>>>>  		if (list_empty(list))
>>>>>>  			bd.last = true;
>>>>>>  		else {
>>>>>> -			struct request *nxt;
>>>>>> -
>>>>>>  			nxt = list_first_entry(list, struct request, queuelist);
>>>>>>  			bd.last = !blk_mq_get_driver_tag(nxt, NULL, false);
>>>>>>  		}
>>>>>>  
>>>>>>  		ret = q->mq_ops->queue_rq(hctx, &bd);
>>>>>>  		if (ret == BLK_STS_RESOURCE) {
>>>>>> +			/*
>>>>>> +			 * If an I/O scheduler has been configured and we got a
>>>>>> +			 * driver tag for the next request already, free it again.
>>>>>> +			 */
>>>>>> +			if (!list_empty(list)) {
>>>>>> +				nxt = list_first_entry(list, struct request, queuelist);
>>>>>> +				blk_mq_put_driver_tag(nxt);
>>>>>> +			}
>>>>> The following way might be more simple and clean:
>>>>>
>>>>> 			if (nxt)
>>>>> 				blk_mq_put_driver_tag(nxt);
>>>>>
>>>>> meantime 'nxt' need to be cleared inside the 'if (list_empty(list))'
>>>>> before .queue_rq().
>>>>
>>>> I had ever thought about that, but to avoid add extra command in the 
>>>> fast path, I made the patch above.
>>>
>>> Got it, so how about changing to the following way simply:
>>>
>>>  			if (nxt && !list_empty(list))
>>>  				blk_mq_put_driver_tag(nxt);
>>>
>> It seems that we even could change it as following:
>>                         if (!list_empty(list))
>>   				blk_mq_put_driver_tag(nxt);
> 
> This is starting to get too clever for its own good, I generally don't
> like to sacrifice readability for performance. In reality, the compiler
> probably figures it out anyway...
> 
> So either make it explicit, or add a nice comment as to why it is the
> way that it is.
> 
yes, it indeed leads to compiler warning of "may be used uninitialized"
maybe the original one could be taken back.
			if (!list_empty(list)) {
				nxt = list_first_entry(list, struct request, queuelist);
				blk_mq_put_driver_tag(nxt);
			}
It is more readable and could avoid the warning.

Thanks
jianchao