diff --git a/racket/src/ChezScheme/mats/thread.ms b/racket/src/ChezScheme/mats/thread.ms
index 51988677a7..2eb9a52f44 100644
--- a/racket/src/ChezScheme/mats/thread.ms
+++ b/racket/src/ChezScheme/mats/thread.ms
@@ -1438,7 +1438,9 @@
                   (if (odd? i)
                       (let loop () (unless (ftype-lock! A (y) a) (printf "waiting\n") (loop)))
                       (ftype-spin-lock! A (y) a))
+                  (memory-order-acquire)
                   (ftype-set! A (x) a ((if (odd? n) + -) (ftype-ref A (x) a) 1.0))
+                  (memory-order-release)
                   (ftype-unlock! A (y) a))))))
         (let loop ()
           (if (equal? (length ls) n)