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

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Message-ID: <ac37c570-d5c7-052c-f2cd-2b05a1e9e03a@oracle.com>
Date:   Wed, 13 Sep 2017 11:59:19 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Jens Axboe <axboe@...nel.dk>, Ming Lei <ming.lei@...hat.com>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blk-mq: put the driver tag of nxt rq before first one is
 requeued



On 09/13/2017 11:54 AM, Jens Axboe wrote:
> On 09/12/2017 09:39 PM, jianchao.wang wrote:
>>> 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.
>>
>> Thanks for your comments , jens and ming. I＇m really appreciative of
>> that.  About the fragility, do you mean the possibility that may
>> release the tag of the next rq which has a driver tag itself (maybe a
>> flush) ?
> 
> I mean that if you do:
> 
> if (!list_empty(list))
> 	blk_mq_put_driver_tag(nxt);
> 
> it's fragile code in the sense that changes elsewhere in the function
> are harder to validate and/or can impact the functionality of that
> simple if and tag put.
> 
> The actual release must always be safe, of course.
> 

Got it, thanks a lot.