diff --git a/activemodel/lib/active_model/attribute_methods.rb b/activemodel/lib/active_model/attribute_methods.rb
index 1ba5b0e3d4d..f336c759d2f 100644
--- a/activemodel/lib/active_model/attribute_methods.rb
+++ b/activemodel/lib/active_model/attribute_methods.rb
@@ -1,4 +1,5 @@
 require 'thread_safe'
+require 'mutex_m'
 
 module ActiveModel
   # Raised when an attribute is not defined.
@@ -333,7 +334,9 @@ module ActiveModel
       end
 
       def generated_attribute_methods #:nodoc:
-        @generated_attribute_methods ||= Module.new.tap { |mod| include mod }
+        @generated_attribute_methods ||= Module.new {
+          extend Mutex_m
+        }.tap { |mod| include mod }
       end
 
       protected
diff --git a/activerecord/lib/active_record/attribute_methods.rb b/activerecord/lib/active_record/attribute_methods.rb
index 609c6e8cab1..7c422ecbe05 100644
--- a/activerecord/lib/active_record/attribute_methods.rb
+++ b/activerecord/lib/active_record/attribute_methods.rb
@@ -1,4 +1,5 @@
 require 'active_support/core_ext/enumerable'
+require 'mutex_m'
 
 module ActiveRecord
   # = Active Record Attribute Methods
@@ -18,12 +19,22 @@ module ActiveRecord
     end
 
     module ClassMethods
+      def inherited(child_class) #:nodoc:
+        child_class.initialize_generated_modules
+        super
+      end
+
+      def initialize_generated_modules # :nodoc:
+        @generated_attribute_methods = Module.new { extend Mutex_m }
+        include @generated_attribute_methods
+      end
+
       # Generates all the attribute related methods for columns in the database
       # accessors, mutators and query methods.
       def define_attribute_methods # :nodoc:
         # Use a mutex; we don't want two thread simultaneously trying to define
         # attribute methods.
-        @attribute_methods_mutex.synchronize do
+        generated_attribute_methods.synchronize do
           return if attribute_methods_generated?
           superclass.define_attribute_methods unless self == base_class
           super(column_names)
diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index f3062435525..7da70cea182 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -88,14 +88,8 @@ module ActiveRecord
     end
 
     module ClassMethods
-      def inherited(child_class) #:nodoc:
-        child_class.initialize_generated_modules
-        super
-      end
-
       def initialize_generated_modules
-        @attribute_methods_mutex = Mutex.new
-
+        super
         # force attribute methods to be higher in inheritance hierarchy than other generated methods
         generated_attribute_methods.const_set(:AttrNames, Module.new {
           def self.const_missing(name)