Re: [PATCH] percpu-rwsem: use barrier in unlock path

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, Oct 18, 2012 at 9:00 AM, Mikulas Patocka <mpatocka@redhat.com> wrote:
>
> What is the procedure for making changes that require support of
> architectures? It is trivial to make a patch that moves this into
> arch-specific includes, the problem is that the patch break all the
> architectures - I wrote support for x86, sparc, parisc, alpha (I can test
> those) but not the others.

We'd need to add it to everybody.

It shouldn't need per-architecture testing - since "smp_mb()" is
always safe. So we could just make all architectures default to that,
and then for x86 (and potentially others that have cheaper models for
release-consistency) just do the optimized one.

We *could* also simply do something like

   #ifndef smp_release_before_store
     #define smp_release_before_store() smp_mb()
   #endif

and basically make the rule be that only architectures that have a
cheaper one need to define it at all. That may be the correct
short-term fix, since there initially would be only a single user.

                  Linus