diff --git a/sys/cddl/compat/opensolaris/sys/atomic.h b/sys/cddl/compat/opensolaris/sys/atomic.h
index 65c9e712459..9b81876455d 100644
--- a/sys/cddl/compat/opensolaris/sys/atomic.h
+++ b/sys/cddl/compat/opensolaris/sys/atomic.h
@@ -117,6 +117,12 @@ atomic_inc_64_nv(volatile uint64_t *target)
 	return (atomic_add_64_nv(target, 1));
 }
 
+static __inline uint64_t
+atomic_dec_64_nv(volatile uint64_t *target)
+{
+	return (atomic_add_64_nv(target, -1));
+}
+
 #if !defined(COMPAT_32BIT) && defined(__LP64__)
 static __inline void *
 atomic_cas_ptr(volatile void *target, void *cmp,  void *newval)