On Thu, 10 Apr 2008 13:22:56 +1000 Benjamin Herrenschmidt <[EMAIL PROTECTED]>
wrote:
> Some architecture need to maintain a kmem cache for thread info
> structures. (next patch adds that to powerpc to fix an alignment
> problem).
>
> There is no good arch callback to use to initialize that cache
> that I can find, so this adds a new one and adds an empty macro
> for when it's not implemented.
>
> Signed-off-by: Benjamin Herrenschmidt <[EMAIL PROTECTED]>
> ---
>
> So we have the choice here between:
>
> - the ifdef on the func name that I did, consistent with what
> I did before for iomap, which iirc Linus liked
>
> - add some more ARCH_HAS_* or HAVE_* (yuck)
>
> - add an empty definition to all archs .h (pain in the neck but I
> can do it, though it will be an annoying patch to keep around)
>
> - do a weak function (will slightly bloat everybody for no good reason)
>
> So unless there is strong complaints, I'd like to stick to my
> current approach.
>
> include/linux/sched.h | 4 ++++
> init/main.c | 1 +
> 2 files changed, 5 insertions(+)
>
> --- linux-work.orig/init/main.c 2008-04-10 13:11:06.000000000 +1000
> +++ linux-work/init/main.c 2008-04-10 13:11:19.000000000 +1000
> @@ -623,6 +623,7 @@ asmlinkage void __init start_kernel(void
> if (efi_enabled)
> efi_enter_virtual_mode();
> #endif
> + thread_info_cache_init();
> fork_init(num_physpages);
> proc_caches_init();
> buffer_init();
> Index: linux-work/include/linux/sched.h
> ===================================================================
> --- linux-work.orig/include/linux/sched.h 2008-04-10 13:11:44.000000000
> +1000
> +++ linux-work/include/linux/sched.h 2008-04-10 13:12:05.000000000 +1000
> @@ -1893,6 +1893,10 @@ static inline unsigned long *end_of_stac
>
> #endif
>
> +#ifndef thread_info_cache_init
> +#define thread_info_cache_init do { } while(0)
> +#endif
This trick does cause a bit of a problem: it is undefined which arch header
file is to provide the alternative definition of thread_info_cache_init.
So we can (and have) ended up in the situation where the override appears
in different files on different architectures and various screwups ensue.
So I'd suggest that we have a bigfatcomment telling implementors which file
the override should be implemented in. And make sure that this arch file is
directly included from within sched.h.
I have a suspicion that we can still get in a mess if .c files include the
per-arch file and don't include sched.h, but I forget where this happened
and why it broke stuff.
Sigh. A nice, coded-in-C implementation within each and every architecture
remains the best implementation, and all the little tricks-to-save-typing
have failure modes.
otoh, if only one .c file will ever call this function then I think that
all problems are solved by
a) moving the above ifdeffery into the .c file
b) adding a comment explaining which arch file must provide the override
c) directly including that file from within the .c file.
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