An apparatus (300) for processing data comprises a plurality of memory access request sources (102,104) which generate memory access requests. Each of the memory access request sources has a local memory (106,108), and the apparatus also includes a shared memory (110). When the memory access requests are atomic memory access requests, contention may arise over common data. When this occurs, the present technique triggers a switch of processing data in the local memory of a memory access request source to processing data in the shared memory.