diff --git a/activesupport/lib/active_support.rb b/activesupport/lib/active_support.rb
index 0f348c3ef4..459c5fccc1 100644
--- a/activesupport/lib/active_support.rb
+++ b/activesupport/lib/active_support.rb
@@ -117,6 +117,10 @@ module ActiveSupport
   def self.utc_to_local_returns_utc_offset_times=(value)
     DateAndTime::Compatibility.utc_to_local_returns_utc_offset_times = value
   end
+
+  def self.current_attributes_use_thread_variables=(value)
+    CurrentAttributes._use_thread_variables = value
+  end
 end
 
 autoload :I18n, "active_support/i18n"
diff --git a/activesupport/lib/active_support/current_attributes.rb b/activesupport/lib/active_support/current_attributes.rb
index cc1a7e92b2..64f3d51114 100644
--- a/activesupport/lib/active_support/current_attributes.rb
+++ b/activesupport/lib/active_support/current_attributes.rb
@@ -143,13 +143,23 @@ module ActiveSupport
         current_instances.clear
       end
 
+      def _use_thread_variables=(value) # :nodoc:
+        @@use_thread_variables = value
+      end
+      @@use_thread_variables = false
+
       private
         def generated_attribute_methods
           @generated_attribute_methods ||= Module.new.tap { |mod| include mod }
         end
 
         def current_instances
-          Thread.current[:current_attributes_instances] ||= {}
+          if @@use_thread_variables
+            Thread.current.thread_variable_get(:current_attributes_instances) ||
+              Thread.current.thread_variable_set(:current_attributes_instances, {})
+          else
+            Thread.current[:current_attributes_instances] ||= {}
+          end
         end
 
         def current_instances_key
diff --git a/activesupport/test/current_attributes_test.rb b/activesupport/test/current_attributes_test.rb
index 1544557f49..0196331d13 100644
--- a/activesupport/test/current_attributes_test.rb
+++ b/activesupport/test/current_attributes_test.rb
@@ -174,4 +174,23 @@ class CurrentAttributesTest < ActiveSupport::TestCase
   test "respond_to? for methods that have not been called" do
     assert_equal true, Current.respond_to?("respond_to_test")
   end
+
+  test "CurrentAttributes use fiber-local variables" do
+    Session.current = 42
+    enumerator = Enumerator.new do |yielder|
+      yielder.yield Session.current
+    end
+    assert_nil enumerator.next
+  end
+
+  test "CurrentAttributes can use thread-local variables" do
+    ActiveSupport::CurrentAttributes._use_thread_variables = true
+    Session.current = 42
+    enumerator = Enumerator.new do |yielder|
+      yielder.yield Session.current
+    end
+    assert_equal 42, enumerator.next
+  ensure
+    ActiveSupport::CurrentAttributes._use_thread_variables = false
+  end
 end