diff --git a/lib/weakref.rb b/lib/weakref.rb
index ee5444a..2882d9f 100644
--- a/lib/weakref.rb
+++ b/lib/weakref.rb
@@ -28,21 +28,19 @@ class WeakRef < Delegator
   @@id_rev_map =  {}            # ref -> obj
   @@mutex = Mutex.new
   @@final = lambda {|id|
-    @@mutex.synchronize {
-      rids = @@id_map[id]
-      if rids
-        for rid in rids
-          @@id_rev_map.delete(rid)
-        end
-        @@id_map.delete(id)
-      end
-      rid = @@id_rev_map[id]
-      if rid
-        @@id_rev_map.delete(id)
-        @@id_map[rid].delete(id)
-        @@id_map.delete(rid) if @@id_map[rid].empty?
+    rids = @@id_map[id]
+    if rids
+      for rid in rids
+        @@id_rev_map.delete(rid)
       end
-    }
+      @@id_map.delete(id)
+    end
+    rid = @@id_rev_map[id]
+    if rid
+      @@id_rev_map.delete(id)
+      @@id_map[rid].delete(id)
+      @@id_map.delete(rid) if @@id_map[rid].empty?
+    end
   }
 
   ##