eagerly allocate the renderer object

this means the reader doesn't need to lock, but does have the added cost
of a new object created for every controller

actionpack/lib/action_controller/base.rb

@@ -248,6 +248,7 @@ def self.without_modules(*modules)
     MODULES.each do |mod|
       include mod
     end
+    setup_renderer!
 
     # Define some internal variables that should not be propagated to the view.
     PROTECTED_IVARS = AbstractController::Rendering::DEFAULT_PROTECTED_INSTANCE_VARIABLES + [

actionpack/lib/action_controller/metal/rendering.rb

@@ -11,20 +11,16 @@ module ClassMethods
       # Documentation at ActionController::Renderer#render
       delegate :render, to: :renderer
 
-      RENDERER_LOCK = Mutex.new
-
-      attr_writer :renderer
-
       # Returns a renderer instance (inherited from ActionController::Renderer)
       # for the controller.
-      def renderer
-        @renderer || RENDERER_LOCK.synchronize do
-          @renderer ||= Renderer.for(self)
-        end
+      attr_reader :renderer
+
+      def setup_renderer! # :nodoc:
+        @renderer = Renderer.for(self)
       end
 
       def inherited(klass)
-        klass.renderer = nil
+        klass.setup_renderer!
         super
       end
     end

actionpack/test/controller/renderer_test.rb

 require 'abstract_unit'
 
 class RendererTest < ActiveSupport::TestCase
+  test 'action controller base has a renderer' do
+    assert ActionController::Base.renderer
+  end
+
   test 'creating with a controller' do
     controller = CommentsController
     renderer   = ActionController::Renderer.for controller