diff --git a/js/src/jsstr.cpp b/js/src/jsstr.cpp
index b3beae68ffdb..925bf319fe6b 100644
--- a/js/src/jsstr.cpp
+++ b/js/src/jsstr.cpp
@@ -104,18 +104,6 @@ js_GetStringChars(JSContext *cx, JSString *str)
 void
 JSString::flatten()
 {
-    // Diagnostic: serialize all calls to this function to see
-    // if concurrent calls are crashing us.
-    JS_LOCK_RUNTIME(asCell()->compartment()->rt);
-    // The main body of this function can be executed only if
-    // the string is a rope. With multiple threads, it's possible
-    // we waited while another one ran, and the string has
-    // already been flattened for us.
-    if (!isRope()) {
-        JS_UNLOCK_RUNTIME(asCell()->compartment()->rt);
-        return;
-    }
-
     JSString *topNode;
     jschar *chars;
     size_t capacity;
@@ -193,8 +181,6 @@ JSString::flatten()
     /* Set null terminator. */
     chars[pos] = 0;
     topNode->initFlatMutable(chars, pos, capacity);
-
-    JS_UNLOCK_RUNTIME(asCell()->compartment()->rt);
 }
 
 #ifdef JS_TRACER