A multi core processor implements a cash coherency protocol in which probe messages are address-ordered on a probe channel while responses are un-ordered on a response channel. When a first core generates a read of an address that misses in the first core's cache, a line fill is initiated. If a second core is writing the same address, the second core generates an update on the addressed ordered probe channel. The second core's update may arrive before or after the first core's line fill returns. If the update arrived before the fill returned, a mask is maintained to indicate which portions of the line were modified by the update so that the late arriving line fill only modifies portions of the line that were unaffected by the earlier-arriving update.