diff --git a/app/controllers/checkout_controller.rb b/app/controllers/checkout_controller.rb
index 71e9239523..77eef00ed4 100644
--- a/app/controllers/checkout_controller.rb
+++ b/app/controllers/checkout_controller.rb
@@ -3,6 +3,8 @@ require 'open_food_network/address_finder'
 class CheckoutController < Spree::CheckoutController
   layout 'darkswarm'
 
+  prepend_around_filter :lock_variants, only: :update
+
   prepend_before_filter :check_hub_ready_for_checkout
   prepend_before_filter :check_order_cycle_expiry
   prepend_before_filter :require_order_cycle
@@ -76,6 +78,33 @@ class CheckoutController < Spree::CheckoutController
 
   private
 
+  # We need locks to avoid a race condition on stock checking. Otherwise we end
+  # up with negative stock when many people check out at the same time. This
+  # implementation makes a checkout wait for all other checkouts containing one
+  # of the order`s variants.
+  #
+  # There are many places in which stock is stored in the database. Row locking
+  # on variant level ensures that there are no conflicts even when an item is
+  # sold through different shops.
+  #
+  # Ordering the variants by id prevents deadlocks. Plucking the id sends the
+  # locking query without building Spree::Variant objects.
+  def lock_variants
+    # The before action `load_order` handles this error state:
+    return yield if current_order.nil?
+
+    ActiveRecord::Base.transaction do
+      # Prevent another checkout from completing the order at the same time:
+      current_order.lock!
+
+      # Prevent any other checkouts of the same line items at the same time:
+      variant_ids = current_order.line_items.select(:variant_id)
+      Spree::Variant.where(id: variant_ids).order(:id).lock.pluck(:id)
+
+      yield
+    end
+  end
+
   def set_default_bill_address
     if params[:order][:default_bill_address]
       new_bill_address = @order.bill_address.clone.attributes