1. A method of data processing in a data processing system having a shared memory system, the method comprising: in a processing unit of the data processing system, executing a memory transaction including a transactional store instruction that causes a processing unit of the data processing system to make a conditional update to a target memory block of the shared memory system conditioned on successful commitment of the memory transaction, wherein executing the memory transaction includes executing a transaction check instruction within the memory transaction; in response to executing the transaction check instruction, the processing unit determining, prior to conclusion of the memory transaction, whether the target memory block of the shared memory system was modified after the conditional update caused by execution of the transactional store instruction; and in response to determining that the target memory block has been modified, setting a condition register within the processing unit to indicate a conflict for the memory transaction.