diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index 4a01c172..a3fcf82a 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -3,6 +3,8 @@ require 'active_model/serializable'
 require 'active_model/serializer/associations'
 require 'active_model/serializer/config'
 
+require 'thread'
+
 module ActiveModel
   class Serializer
     include Serializable
@@ -14,7 +16,9 @@ module ActiveModel
       end
 
       def setup
-        yield CONFIG
+        Mutex.new.synchronize do
+          yield CONFIG
+        end
       end
 
       def embed(type, options={})
diff --git a/lib/active_model/serializer/config.rb b/lib/active_model/serializer/config.rb
index 8a7ef80c..fb204867 100644
--- a/lib/active_model/serializer/config.rb
+++ b/lib/active_model/serializer/config.rb
@@ -1,41 +1,31 @@
-require 'thread'
-
 module ActiveModel
   class Serializer
     class Config
-      def initialize
-        @data = {}
-        @mutex = Mutex.new
+      def initialize(data = {})
+        @data = data
       end
 
       def each(&block)
-        @mutex.synchronize do
-          @data.each(&block)
-        end
+        @data.each(&block)
       end
 
       def clear
-        @mutex.synchronize do
-          @data.clear
-        end
+        @data.clear
       end
 
       def method_missing(name, *args)
-        @mutex.synchronize do
-          name = name.to_s
-          return @data[name] if @data.include?(name)
-          match = name.match(/\A(.*?)([?=]?)\Z/)
-          case match[2]
-          when "="
-            @data[match[1]] = args.first
-          when "?"
-            !!@data[match[1]]
-          end
+        name = name.to_s
+        return @data[name] if @data.include?(name)
+        match = name.match(/\A(.*?)([?=]?)\Z/)
+        case match[2]
+        when "="
+          @data[match[1]] = args.first
+        when "?"
+          !!@data[match[1]]
         end
       end
     end
 
-    CONFIG = Config.new
-    CONFIG.embed = :objects
+    CONFIG = Config.new('embed' => :objects) # :nodoc:
   end
 end