diff -r 853a8cb902aa -r e557a9ddee5f src/Pure/Concurrent/synchronized.ML
--- a/src/Pure/Concurrent/synchronized.ML	Fri Dec 12 14:30:33 2014 +0100
+++ b/src/Pure/Concurrent/synchronized.ML	Fri Dec 12 14:31:57 2014 +0100
@@ -33,7 +33,8 @@
   cond = ConditionVar.conditionVar (),
   var = Unsynchronized.ref x};
 
-fun value (Var {var, ...}) = ! var;
+fun value (Var {name, lock, var, ...}) =
+  Multithreading.synchronized name lock (fn () => ! var);
 
 
 (* synchronized access *)