quoted

+#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.

I this case it's well defined: thread_info.h. Maybe I should add a
comment ?

So we can (and have) ended up in the situation where the override appears
in different files on different architectures and various screwups ensue.

Yup.

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.

Will do.

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.

In this case, there's only one call site and will only every be one, so
that shouldn't be a problem. I don't see init/main.c not including
sched.h

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.

Well, I started doing it in all arch, and people around here told me
that was not a good idea , that it would be trouble if the prototype
ever had to change (adding an arg, etc... though very unlikely to happen
in that case, granted).

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.

I can definitely do that. I have no problem either way. I can add to all
archs too, it's just that whatever way I choose, some people won't be
happy with it :-)

Anyway, I'll move the ifdeferry to init/main.c then.

Cheers,
Ben.