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

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Date:   Tue, 12 Sep 2017 21:54:06 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     "jianchao.wang" <jianchao.w.wang@...cle.com>,
        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 09/12/2017 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.

-- 
Jens Axboe