A data processing apparatus and corresponding method of data processing are provided. The data processing apparatus comprises a temporary data store configured to store data items retrieved from a memory, wherein the temporary data store selects one of its plural data storage locations in which to store a newly retrieved data item according to a predetermined circular sequence. An index data store is configured to store index items corresponding to the data items stored in the temporary data store, wherein presence of a valid index item in the index data store is indicative of a corresponding data item in the temporary data store. Invalidation control circuitry performs a rolling invalidation process with respect to the index items stored in the index data store, comprising sequentially processing the index items stored in the index data store and selectively marking the index items as invalid according to a predetermined criterion.