linux-kernel - Re: block-rbd: One function call less in rbd_dev_probe

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Message-ID: <CAOi1vP-p8jSee8hEprH1eowDY4daSR0zjTgDpbyF6=vJRXTr1g@mail.gmail.com>
Date:	Thu, 26 Nov 2015 12:37:08 +0100
From:	Ilya Dryomov <idryomov@...il.com>
To:	SF Markus Elfring <elfring@...rs.sourceforge.net>
Cc:	Alex Elder <elder@...nel.org>, Sage Weil <sage@...hat.com>,
	Ceph Development <ceph-devel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	kernel-janitors@...r.kernel.org,
	Julia Lawall <julia.lawall@...6.fr>
Subject: Re: block-rbd: One function call less in rbd_dev_probe_parent() after
 error detection

On Thu, Nov 26, 2015 at 8:54 AM, SF Markus Elfring
<elfring@...rs.sourceforge.net> wrote:
>>> I interpreted the eventual passing of a null pointer to the rbd_dev_destroy()
>>> function as an indication for further source code adjustments.
>>
>> If all error paths could be adjusted so that NULL pointers are never passed in,
>> destroy functions wouldn't need to have a NULL check, would they?
>
> How do you think about to clarify corresponding implementation details a bit more?
>
> * Why was the function "rbd_dev_probe_parent" implemented in the way
>   that it relies on a sanity check in the function "rbd_dev_destroy" then?

Because it's not a bad thing?  What's wrong with an init to NULL,
a possible assignment, in this case from rbd_dev_create(), and an
unconditional rbd_dev_destroy()?

The NULL check in rbd_dev_destroy() is not a sanity check, it's
a feature.  It's not there to "fixup" callers that pass NULL - it's
there because it is _expected_ that some callers will pass NULL.

> * How are the chances to restructure the source code a bit (like changing a few
>   jump labels) so that it should also work without an extra function call
>   during error handling there?

As I said in my reply to Dan, the problem with rbd_dev_probe_parent()
is the calling code which expects it to call unparent if ->parent_spec.
This makes it stand out and confuses people, but can't be fixed without
refactoring a bunch of other code.

The extra function call is *not* a problem.

Thanks,

                Ilya
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