diff --git a/app/controllers/admin/bulk_line_items_controller.rb b/app/controllers/admin/bulk_line_items_controller.rb
index 3faf8151e6..9f521f1c52 100644
--- a/app/controllers/admin/bulk_line_items_controller.rb
+++ b/app/controllers/admin/bulk_line_items_controller.rb
@@ -16,6 +16,10 @@ module Admin
       load_line_item
       authorize_update!
 
+      # `with_lock` acquires an exclusive row lock on order so no other
+      # requests can update it until the transaction is commited.
+      # See https://github.com/rails/rails/blob/3-2-stable/activerecord/lib/active_record/locking/pessimistic.rb#L69
+      # and https://www.postgresql.org/docs/current/static/sql-select.html#SQL-FOR-UPDATE-SHARE
       order.with_lock do
         if @line_item.update_attributes(params[:line_item])
           order.update_distribution_charge!
diff --git a/app/models/spree/order_decorator.rb b/app/models/spree/order_decorator.rb
index d0c38e8753..8c0eddfea0 100644
--- a/app/models/spree/order_decorator.rb
+++ b/app/models/spree/order_decorator.rb
@@ -181,6 +181,10 @@ Spree::Order.class_eval do
   end
 
   def update_distribution_charge!
+    # `with_lock` acquires an exclusive row lock on order so no other
+    # requests can update it until the transaction is commited.
+    # See https://github.com/rails/rails/blob/3-2-stable/activerecord/lib/active_record/locking/pessimistic.rb#L69
+    # and https://www.postgresql.org/docs/current/static/sql-select.html#SQL-FOR-UPDATE-SHARE
     with_lock do
       EnterpriseFee.clear_all_adjustments_on_order self