Add the atomic_thread_fence() family of functions with intent to

provide a semantic defined by the C11 fences with corresponding
memory_order.

atomic_thread_fence_acq() gives r | r, w, where r and w are read and
write accesses, and | denotes the fence itself.

atomic_thread_fence_rel() is r, w | w.

atomic_thread_fence_acq_rel() is the combination of the acquire and
release in single operation.  Note that reads after the acq+rel fence
could be made visible before writes preceeding the fence.

atomic_thread_fence_seq_cst() orders all accesses before/after the
fence, and the fence itself is globally ordered against other
sequentially consistent atomic operations.

Reviewed by:	alc
Discussed with:	bde
Sponsored by:	The FreeBSD Foundation
MFC after:	3 weeks

1 files changed, 32 insertions, 0 deletions

diff --git a/sys/amd64/include/atomic.h b/sys/amd64/include/atomic.h
index 6d5c6b6..7892d51 100644
--- a/sys/amd64/include/atomic.h
+++ b/sys/amd64/include/atomic.h
@@ -84,6 +84,10 @@ u_int	atomic_fetchadd_int(volatile u_int *p, u_int v);
 u_long	atomic_fetchadd_long(volatile u_long *p, u_long v);
 int	atomic_testandset_int(volatile u_int *p, u_int v);
 int	atomic_testandset_long(volatile u_long *p, u_int v);
+void	atomic_thread_fence_acq(void);
+void	atomic_thread_fence_acq_rel(void);
+void	atomic_thread_fence_rel(void);
+void	atomic_thread_fence_seq_cst(void);
 
 #define	ATOMIC_LOAD(TYPE)					\
 u_##TYPE	atomic_load_acq_##TYPE(volatile u_##TYPE *p)
@@ -328,6 +332,34 @@ atomic_store_rel_##TYPE(volatile u_##TYPE *p, u_##TYPE v)	\
 }								\
 struct __hack
 
+static __inline void
+atomic_thread_fence_acq(void)
+{
+
+	__compiler_membar();
+}
+
+static __inline void
+atomic_thread_fence_rel(void)
+{
+
+	__compiler_membar();
+}
+
+static __inline void
+atomic_thread_fence_acq_rel(void)
+{
+
+	__compiler_membar();
+}
+
+static __inline void
+atomic_thread_fence_seq_cst(void)
+{
+
+	__storeload_barrier();
+}
+
 #endif /* KLD_MODULE || !__GNUCLIKE_ASM */
 
 ATOMIC_ASM(set,	     char,  "orb %b1,%0",  "iq",  v);