Re: __weak vs ifdef

Grant Likely
2010-02-18T02:07:57Z

Reaching back into an old discussion....

On Sat, Jul 26, 2008 at 12:38 PM, Linus Torvalds
<torvalds@linux-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.