linux-kernel - Re: __weak vs ifdef

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Message-ID: <fa686aa41002171807s71ac7176l769d78e2bb5d9a1@mail.gmail.com>
Date:	Wed, 17 Feb 2010 19:07:34 -0700
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Matthew Wilcox <matthew@....cx>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Adrian Bunk <bunk@...nel.org>,
	Andrea Righi <righi.andrea@...il.com>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: __weak vs ifdef

Reaching back into an old discussion....

On Sat, Jul 26, 2008 at 12:38 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
>
> On Fri, 25 Jul 2008, Matthew Wilcox wrote:
>
>> On Fri, Jul 25, 2008 at 02:34:55AM -0700, Andrew Morton wrote:
>> > We should make arch_pick_mmap_layout __weak and nuke that ifdef.
>>
>> I strongly disagree.  I find it makes it harder to follow code flow
>> when __weak functions are involved.  Ifdefs are ugly, no question, but
>> they're easier to grep for
>
> Hell no, they're not.
>
> Our use of random HAVE_ARCH_xyz or ARCH_SUPPORTS_xyz etc stuff makes
> things _totally_ impossible to grep for.
>
> In contrast, it we did this code as
>
>        #ifndef arch_pick_mmap_layout
>        void __weak arch_pick_mmap_layout(struct mm_struct *mm)
>        {
>                mm->mmap_base = TASK_UNMAPPED_BASE;
>                mm->get_unmapped_area = arch_get_unmapped_area;
>                mm->unmap_area = arch_unmap_area;
>        }
>        #endif
>
> then trying to grep for arch_pick_mmap_layout() would show EVERY SINGLE
> RELEVANT CASE! And it would show the "__weak" there too, so that once
> people get used to this convention, they'd have a really easy time
> figuring out the rules from just the output of the 'grep'.
[...]

Question.  If I use this pattern, and use the __weak attribute on core
code functions wrapped with a #ifndef, then how does it mesh with
EXPORT_SYMBOL*() statements?  Do both the core code, and the arch
override need to do EXPORT_SYMBOL(), or should EXPORT_SYMBOL() only
appear at the core code site?

I also assume that at the core code site, the EXPORT_SYMBOL() must
appear inside the #ifndef block so that a #define override doesn't
break things.  Correct?

Cheers,
g.

-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.
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