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

Jens Axboe
2017-09-13T03:54:06Z

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