1. A method of data processing in a data processing system, the method comprising: a processor core executing a code segment including a memory transaction and a non-transactional memory access instructions preceding the memory transaction in program order, wherein: the memory transaction includes at least an initiating instruction, a transactional memory access instruction, and a terminating instruction; the initiating instruction has an implicit barrier that imparts an effect of ordering execution of the transactional memory access instruction within the memory transaction with respect to the non-transactional memory access instructions preceding the memory transaction in program order; and wherein the executing includes: executing the transactional memory access instruction within the memory transaction concurrently with at least one of the non-transactional memory access instructions preceding the memory transaction in program order; and enforcing the barrier implicit in the initiating instruction following execution of the initiating instruction only if the memory transaction commits successfully to a shared memory system included in data processing system.