Signalling regions: Multiprocessing in a shared memory reconsidered

The goal of this paper is to return attention to two problems that arise in the context of supporting the monitor as a mechanism for concurrent programming. This paper will re‐examine the monitor concept in its original context—a multiprocessing environment implemented on a single processor sharing memory with and being interrupted by asynchronous peripheral devices—and will address the two previously unresolved problems. The first is the conflict between the immediate resumption requirement in explicit signalling and the policies and priorities of the process scheduler. The second is the possibility of deadlock inherent in nested monitors and in its most important instance, the dynamic resource allocation problem. After briefly describing the historical context of these two problems, the paper proposes a language structure called a signalling region that together with the notion of encapsulation by modules solves the immediate resumption problem and avoids the nested monitor problem. The former is done by a combination of the signal‐and‐return semantics of Concurrent Pascal and the signal‐and‐continue semantics of Mesa and StarMod. The latter is done by suggesting that mutual exclusion and data encapsulation are distinct concepts that, if separated, make nested encapsulation possible while avoiding the problems of nested mutual exclusion. Classical examples of the use of signalling regions in an extended Modula‐2 are given as well as an implementation by translation to unextended Modula‐2 together with a Kernel module.