[PATCH v3 0/6] atomics: powerpc: Implement relaxed/acquire/release variants of some atomics

Boqun Feng
2015-10-12T14:14:51Z

Hi,

This is v3 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

Paul, Peter and Will, thank you all for the comments and suggestions,
that's really a lot of fun to discuss these with you and very
enlightening to me ;-)

Changes since v2:

* reorder the patches to put the fix of cmpxchg, xchg and their
atomic_ versions first, and Cc stable. (Peter Zijlstra)

* modify the commit log for implementation of cmpxchg family to
explain why we implement some operation using assembly code
(Peter Zijlstra)

* add implementation and test for {inc,dec}_return atomics.
(Will Deacon)

* rebase on current locking/core branch of tip tree (commit
00eb4bab69db3)

* rewrite the macros for generating tests to save some lines of
code

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