A cache coherent data processing system includes at least first and second coherency domains. The first coherency domain includes a system memory controller for a system memory and a first processing unit having a first cache memory. The second coherency domain includes a second processing unit having a second cache memory. In the first cache memory, a coherency state field associated with a storage location and an address tag is set to a first coherency state. In response to snooping an exclusive access request specifying a target address matching the address tag, the first cache memory provides a first partial response to the exclusive access request based at least in part upon the first coherency state. In response to snooping the exclusive access request, the memory controller determines whether it is responsible for the target address and provides a second partial response to the exclusive access request based at least in part upon an outcome of the determination. At least the first and second partial responses are accumulated to obtain a combined response for the exclusive access request. The combined response includes an indication of whether or not a highest point of coherency and a memory controller of a home system memory for the target address reside within a same coherency domain. The first cache memory updates the coherency state field from the first coherency state to a second coherency state in response to the indication in the combined response.