1 files changed, 5 insertions, 2 deletions
diff --git a/activesupport/lib/active_support/notifications/fanout.rb b/activesupport/lib/active_support/notifications/fanout.rb
index 2e5bcf4639..7588fdb67c 100644
--- a/activesupport/lib/active_support/notifications/fanout.rb
+++ b/activesupport/lib/active_support/notifications/fanout.rb
@@ -1,4 +1,5 @@
 require 'mutex_m'
+require 'thread_safe'
 
 module ActiveSupport
   module Notifications
@@ -11,7 +12,7 @@ module ActiveSupport
 
       def initialize
         @subscribers = []
-        @listeners_for = {}
+        @listeners_for = ThreadSafe::Cache.new
         super
       end
 
@@ -44,7 +45,9 @@ module ActiveSupport
       end
 
       def listeners_for(name)
-        synchronize do
+        # this is correctly done double-checked locking (ThreadSafe::Cache's lookups have volatile semantics)
+        @listeners_for[name] || synchronize do
+          # use synchronisation when accessing @subscribers
           @listeners_for[name] ||= @subscribers.select { |s| s.subscribed_to?(name) }
         end
       end