[RFC 2/5] atomics: introduce arch_atomic_op_{acquire, release, fence} helpers

[Boqun Feng]

Some architectures may have their special barriers for acquire, release
and fence semantics, general memory barriers(smp_mb__*_atomic()) in
__atomic_op_*() may be too strong, so arch_atomic_op_*() helpers are
introduced for architectures to provide their own version helpers to
build different variants based on _relaxed variants.

Signed-off-by: Boqun Feng <boqun.feng at gmail.com>
---
 include/linux/atomic.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index 00a5763..622255b 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -34,20 +34,33 @@
  * The idea here is to build acquire/release variants by adding explicit
  * barriers on top of the relaxed variant. In the case where the relaxed
  * variant is already fully ordered, no additional barriers are needed.
+ *
+ * Besides, if an arch has a special barrier for acquire/release, it could
+ * implement its own arch_atomic_op_* and use the same framework for building
+ * variants
  */
+#ifndef arch_atomic_op_acquire
 #define __atomic_op_acquire(op, args...)				\
 ({									\
 	typeof(op##_relaxed(args)) __ret  = op##_relaxed(args);		\
 	smp_mb__after_atomic();						\
 	__ret;								\
 })
+#else
+#define __atomic_op_acquire arch_atomic_op_acquire
+#endif
 
+#ifndef arch_atomic_op_release
 #define __atomic_op_release(op, args...)				\
 ({									\
 	smp_mb__before_atomic();					\
 	op##_relaxed(args);						\
 })
+#else
+#define __atomic_op_release arch_atomic_op_release
+#endif
 
+#ifndef arch_atomic_op_fence
 #define __atomic_op_fence(op, args...)					\
 ({									\
 	typeof(op##_relaxed(args)) __ret;				\
@@ -56,6 +69,9 @@
 	smp_mb__after_atomic();						\
 	__ret;								\
 })
+#else
+#define __atomic_op_fence arch_atomic_op_fence
+#endif
 
 /* atomic_add_return_relaxed */
 #ifndef atomic_add_return_relaxed
-- 
2.5.0