1. A method of data processing in a multiprocessor data processing system including at least first and second coherency domains, wherein the first coherency domain includes at least one processing unit, a system memory and a cache memory, the method comprising: buffering a cache line in a data array of the cache memory; setting a state field in a cache directory of the cache memory to a coherency state to indicate that the cache line is valid in the data array, that the cache line is held in the cache memory non-exclusively, and that another cache in said second coherency domain may hold a copy of the cache line.