1. A method of data processing in a data processing system, the method comprising: at a cache memory of a data processing system, receiving a transactional memory access request generated by execution by a processor core of a transactional memory access instruction within a requesting memory transaction, wherein the transactional memory access request includes a target address of data to be accessed and indicates a priority of the requesting memory transaction; in response to the transactional memory access request, transactional memory logic detecting a conflict for the target address between the transactional memory access request and a transaction footprint of an existing memory transaction; the transactional memory logic accessing a priority of the existing memory transaction; and in response to detecting the conflict, the transactional memory logic resolving the conflict by causing the cache memory to fail one of the requesting memory transaction and the existing memory transaction based at least in part on the relative priorities of the requesting memory transaction and the existing memory transaction, wherein resolving the conflict includes at least: causing the cache memory to fail the existing memory transaction when the requesting memory transaction has a higher priority than the existing memory transaction, the transactional memory access request is a transactional load request, and the target address is within a store footprint of the existing memory transaction.