[PATCH tip/locking/core v4 0/6] atomics: powerpc: Implement relaxed/acquire/release variants of some atomics

[Boqun Feng]

Hi all,

This is v4 of the series.

Link for v1: https://lkml.org/lkml/2015/8/27/798
Link for v2: https://lkml.org/lkml/2015/9/16/527
Link for v3: https://lkml.org/lkml/2015/10/12/368

Changes since v3:

*	avoid to introduce smp_acquire_barrier__after_atomic()
	(Will Deacon)

*	explain a little bit why we don't implement cmpxchg_release
	with assembly code (Will Deacon)


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"

and {inc,dec}_return has been introduced by commit:

"locking/asm-generic: Add _{relaxed|acquire|release}() variants for
inc/dec atomics"

Both of these are in the current locking/core branch of the tip tree.

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 fix the order guarantee of cmpxchg, xchg and
their atomic_ versions and implements the relaxed/acquire/release
variants based on powerpc memory model and specific barriers, Some
trivial tests for these new variants are 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 6 parts:

1.	Make xchg, cmpxchg and their atomic_ versions a full barrier

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

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

4.	Implement atomic{,64}_{add,sub,inc,dec}_return_* variants

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

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


This patchset is based on current locking/core branch of the tip tree
and all patches are built and boot tested for little endian pseries, and
also tested by 0day.


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

Regards,
Boqun