diff --git a/src/runtime/mgcsweep.go b/src/runtime/mgcsweep.go
index 84b6181..812ba60 100644
--- a/src/runtime/mgcsweep.go
+++ b/src/runtime/mgcsweep.go
@@ -392,7 +392,7 @@
 		// Nobody cares about the credit. Avoid the atomic.
 		return
 	}
-	xadd64(&mheap_.spanBytesAlloc, -int64(unusableBytes))
+	atomic.Xadd64(&mheap_.spanBytesAlloc, -int64(unusableBytes))
 }
 
 func dumpFreeList(s *mspan) {