Index: sys/sys/refcount.h
===================================================================
--- sys/sys/refcount.h
+++ sys/sys/refcount.h
@@ -55,6 +55,27 @@
 	atomic_add_int(count, 1);
 }
 
+/*
+ * This functions returns true if the refcount was incremented. Else
+ * false is returned.
+ */
+static __inline __result_use_check bool
+refcount_acquire_unless_zero(volatile u_int *count)
+{
+	u_int c = *count;
+
+	KASSERT(c < UINT_MAX, ("refcount %p overflowed", count));
+
+	/* add one unless zero */
+	for (;;) {
+		if (c == 0)
+			break;
+		if (atomic_fcmpset_int(count, &c, c + 1))
+			break;
+	}
+	return (c != 0);
+}
+
 static __inline int
 refcount_release(volatile u_int *count)
 {