Coherent attached processor proxy supporting coherence state update in presence of dispatched master

A method of data processing, comprising:

• at a coherent attached processor proxy (CAPP) of a primary coherent system, receiving a memory access request from an attached processor (AP) external to the primary coherent system, wherein the CAPP includes a CAPP directory of contents of a cache memory in the AP that holds copies of memory blocks belonging to a coherent address space of the primary coherent system;
• in response to receiving the memory access request at the CAPP, the CAPP performing a first determination of a coherence state for the target address with respect to the CAPP and allocating a master machine of the CAPP to service the memory access request in accordance with the first determination;
• and
• thereafter, during allocation of the master machine to service the memory access request:
• a snoop machine of the CAPP causing an update to the coherence state for the target address with respect to the CAPP;
• and
• the CAPP performing a second determination of the coherence state for the target address with respect to the CAPP and the master machine in the CAPP servicing the memory access request in accordance with the second determination.