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

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Date:	Thu, 26 Nov 2015 14:41:32 +0100
From:	SF Markus Elfring <elfring@...rs.sourceforge.net>
To:	Ilya Dryomov <idryomov@...il.com>
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

>> * 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?

There are different opinions about this implementation detail.

> What's wrong with an init to NULL, a possible assignment, in this case
> from rbd_dev_create(), and an unconditional rbd_dev_destroy()?

Does this approach look like it is affected by a "one error jump
label bug" symptom?

> The NULL check in rbd_dev_destroy() is not a sanity check,
> it's a feature.

I have got an other impression here.

> It's not there to "fixup" callers that pass NULL

It seems that the explanations on the detail why a function tolerates
passed null pointers can also be different.

> - it's there because it is _expected_ that some callers will pass NULL.

I find it still unnecessary to let a called function like "rbd_dev_destroy"
to repeat the check when you know already that the passed variable contains
a null pointer.

> 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.

I would appreciate if the discussed function could be also improved by itself.
More refactoring might follow at other source code places later.

> The extra function call is *not* a problem.

How many software developers and reviewers will care if corresponding
error handling can also become a bit more efficient?

Regards,
Markus
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