Add generic support for relaxed atomics

Hello,

Here is version three of the patches previously posted here:

  v1: https://lwn.net/Articles/650862/
  v2: https://lwn.net/Articles/651293/

The series adds support for a family of relaxed atomics to the kernel.
More specifically:

  - acquire/release/relaxed flavours of xchg, cmpxchg and {add,sub}_return
  - atomic_read_acquire
  - atomic_set_release

This came out of a separate patch series porting the (barrier-heavy)
qrwlock code to arm64. Rather than have arch-specific hooks littered
around the place, it makes more sense to define a core set of relaxed
atomics that can be used regardless of architecture.

Changes since v2 include:

  - Re-organised the series so it's more amenable to getting merged
    (i.e. included an extra qrwlock dependency)

  - Added some comments to the qrwlock code

  - Added extra comments to delimit the relaxed atomic macros

  - Fixed typo in comment message

Build tested on ARM, arm64, PowerPC and x86.

All feedback welcome,

Will

--->8

Will Deacon (8):
  atomics: add acquire/release/relaxed variants of some atomic
    operations
  asm-generic: rework atomic-long.h to avoid bulk code duplication
  asm-generic: add relaxed/acquire/release variants for atomic_long_t
  lockref: remove homebrew cmpxchg64_relaxed macro definition
  locking/qrwlock: implement queue_write_unlock using smp_store_release
  locking/qrwlock: make use of acquire/release/relaxed atomics
  include/llist: use linux/atomic.h instead of asm/cmpxchg.h
  ARM: atomics: define our SMP atomics in terms of _relaxed operations

 arch/arm/include/asm/atomic.h     |  37 ++---
 arch/arm/include/asm/cmpxchg.h    |  47 +-----
 arch/x86/include/asm/qrwlock.h    |  10 --
 include/asm-generic/atomic-long.h | 263 +++++++++++--------------------
 include/asm-generic/qrwlock.h     |  22 +--
 include/linux/atomic.h            | 323 ++++++++++++++++++++++++++++++++++++++
 include/linux/llist.h             |   2 +-
 kernel/locking/qrwlock.c          |  23 ++-
 lib/lockref.c                     |   8 -
 9 files changed, 463 insertions(+), 272 deletions(-)

-- 
2.1.4

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