Subject: Re: [PATCH] blk-mq: put the driver tag of nxt rq before first one is
 requeued
To: "jianchao.wang" <jianchao.w.wang@oracle.com>,
        Ming Lei <ming.lei@redhat.com>
Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
References: <1505236475-9209-1-git-send-email-jianchao.w.wang@oracle.com>
 <20170912102302.GC15792@ming.t460p>
 <23b700aa-ed37-1081-d980-4073804f293b@oracle.com>
 <20170913012452.GA24642@ming.t460p>
 <4b7dcb10-9ec5-9a00-8d16-009e83ef7b5b@oracle.com>
 <ccda7d87-a0b9-8292-3cb7-8edcca9c7f69@kernel.dk>
 <96a21c69-cf79-58a2-2c71-2e440abf9b36@oracle.com>
From: Jens Axboe <axboe@kernel.dk>
Message-ID: <6327de01-0bf4-7e79-59ad-f6489378bfd5@kernel.dk>
Date: Tue, 12 Sep 2017 20:45:19 -0600
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On 09/12/2017 08:42 PM, jianchao.wang wrote:
> 
> 
> 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.

Exactly, and especially the readability is the key element here. It's
just not worth it to try and be too clever, especially not for something
like this. When you read the above, you immediately know what the code
does without needing a comment. That's not true for the other construct.
You both have to read other parts of the function to figure out what it
does, AND read the entire function to ensure it always does the right
thing. Fragile.

-- 
Jens Axboe