1 files changed, 12 insertions, 10 deletions
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
index f44cba3978..d4649102df 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
@@ -290,17 +290,19 @@ connection.  For example: ActiveRecord::Base.connection.close
       private
 
       def release(conn)
-        thread_id = nil
-
-        if @reserved_connections[current_connection_id] == conn
-          thread_id = current_connection_id
-        else
-          thread_id = @reserved_connections.keys.find { |k|
-            @reserved_connections[k] == conn
-          }
-        end
+        synchronize do
+          thread_id = nil
+
+          if @reserved_connections[current_connection_id] == conn
+            thread_id = current_connection_id
+          else
+            thread_id = @reserved_connections.keys.find { |k|
+              @reserved_connections[k] == conn
+            }
+          end
 
-        @reserved_connections.delete thread_id if thread_id
+          @reserved_connections.delete thread_id if thread_id
+        end
       end
 
       def new_connection