 Solution

With a synchronous reply, send(client,&answer) blocks until the client receives. An indefinitely unready client therefore stalls the smoothing operating-system process, which may in turn stall the timer service and its other clients as in the preceding part. If the client is merely delayed, this is blocking and delayed service; a deadlock requires a circular wait or a client that will never receive.

Requiring correctly used synchronous send-and-receive helps. A client calling sendrec(SMOOTH,&m) is committed to waiting for the matching reply as soon as its request has been accepted. This closes the scheduling window between a separate send and receive and prevents unrelated client computation from postponing that receive. It does not by itself fix slow measurements, arbitrary failures, circular dependencies between other services, or lack of CPU scheduling. With buffered replies the initial effect may instead be queue growth, depending on the message passing contract.