[RFC v2 0/7] atomics: powerpc: Implement relaxed/acquire/release variants of some atomics

[Boqun Feng <boqun.feng@gmail.com>]

Link for v1: https://lkml.org/lkml/2015/8/27/798

Changes since v1:

*	avoid to introduce macro arch_atomic_op_*()

*	also fix the problem that cmpxchg, xchg and their atomic_
	versions are not full barriers in PPC implementation.

*	rebase on v4.3-rc1


Relaxed/acquire/release variants of atomic operations {add,sub}_return
and {cmp,}xchg are introduced by commit:

"atomics: add acquire/release/relaxed variants of some atomic operations"

By default, the generic code will implement a relaxed variant as a full
ordered atomic operation and release/acquire a variant as a relaxed
variant with a necessary general barrier before or after.

On powerpc, which has a weak memory order model, a relaxed variant can
be implemented more lightweightly than a full ordered one. Further more,
release and acquire variants can be implemented with arch-specific
lightweight barriers.

Besides, cmpxchg, xchg and their atomic_ versions are only RELEASE+ACQUIRE
rather that full barriers in current PPC implementation, which is incorrect
according to memory-barriers.txt.

Therefore this patchset implements the relaxed/acquire/release variants
based on powerpc memory model and specific barriers, and fix cmpxchg,
xchg and their atomic_ versions. A trivial test for these new variants
is also included in this series, because some of these variants are not
used in kernel for now, I think is a good idea to at least generate the
code for these variants somewhere.

The patchset consists of 7 parts:

1.	Add trivial tests for the new variants in lib/atomic64_test.c

2.	Allow architectures to define their own __atomic_op_*() helpers
	to build other variants based on relaxed.

3.	Implement atomic{,64}_{add,sub}_return_* variants

4.	Implement xchg_* and atomic{,64}_xchg_* variants

5.	Implement cmpxchg_* atomic{,64}_cmpxchg_* variants

6.	Make atomic{,64}_xchg and xchg full barriers

7.	Make atomic{,64}_cmpxchg and cmpxchg full barriers

This patchset is based on v4.3-rc1 and all patches are built and boot
tested for little endian powernv and pseries, and also tested by 0day.


Looking forward to any suggestion, question and comment ;-)

Regards,
Boqun