A multiprocessor data processing system is described wherein the processors communicate to each other via a shared memory. Each of the processors comprises an administration unit (18a) and a computational unit. The administration unit of a writing processor maintains information defining a section in the memory which is free for storing data objects for readout by the reading processor. The administration unit of the reading processor maintains information defining a section in the memory in which the writing processor has written completed data for the data objects. The processors are arranged to signal a message to another processor via a processor synchronization channel for updating the information in the administration unit of said other processor.