Index: arch/amd64/include/atomic.h
===================================================================
RCS file: /cvs/src/sys/arch/amd64/include/atomic.h,v
retrieving revision 1.18
diff -u -p -r1.18 atomic.h
--- arch/amd64/include/atomic.h	17 Aug 2016 11:56:42 -0000	1.18
+++ arch/amd64/include/atomic.h	3 May 2017 01:20:10 -0000
@@ -263,10 +263,25 @@ _atomic_sub_long_nv(volatile unsigned lo
 #define __membar(_f) do { __asm __volatile(_f ::: "memory"); } while (0)
 
 #ifdef MULTIPROCESSOR
-#define membar_enter()		__membar("mfence")
-#define membar_exit()		__membar("")
-#define membar_producer()	__membar("")
-#define membar_consumer()	__membar("")
+
+static inline void
+__membar_store(void)
+{
+	unsigned long word;
+	__asm __volatile("movq $0,%0" : "=m" (word) : : "memory");
+}
+
+static inline void
+__membar_loadstore(void)
+{
+	unsigned long word;
+	__asm __volatile("lock addq $0,%0" : "=m" (word) : : "memory");
+}
+
+#define membar_enter()		__membar_loadstore()
+#define membar_exit()		__membar_store()
+#define membar_producer()	__membar_store()
+#define membar_consumer()	__membar_loadstore()
 #define membar_sync()		__membar("mfence")
 #else
 #define membar_enter()		__membar("")