eagerly initialize the attributes module to avoid check-then-set race conditions

1 files changed, 4 insertions, 1 deletions

diff --git a/activemodel/lib/active_model/attribute_methods.rb b/activemodel/lib/active_model/attribute_methods.rb
index 1ba5b0e3d4..f336c759d2 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