diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index e4e66d61..260d3598 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -4,15 +4,12 @@ require 'active_model/serializer/associations'
 require 'active_model/serializer/configuration'
 require 'active_model/serializer/dsl'
 
-require 'thread'
 require 'forwardable'
 
 module ActiveModel
   class Serializer
     include Serializable
 
-    @mutex = Mutex.new
-
     class << self
       def inherited(base)
         base._root = _root
@@ -20,12 +17,6 @@ module ActiveModel
         base._associations = (_associations || {}).dup
       end
 
-      def setup
-        @mutex.synchronize do
-          yield Configuration.global
-        end
-      end
-
       def embed(type, options={})
         Configuration.global.embed = type
         Configuration.global.embed_in_root = true if options[:embed_in_root] || options[:include]