An application of temporal projection to interleaving concurrency

Abstract

We revisit the earliest temporal projection operator Π in discrete-time Propositional Interval Temporal Logic (PITL) and use it to formalise interleaving concurrency. The logical properties of Π as a normal modality and a way to eliminate it in both PITL and conventional point-based Linear-Time Temporal Logic (LTL), which can be viewed as a PITL subset, are examined, as are stutter-invariant formulas. Striking similarities between the expressiveness of Π and the standard LTL operator U (‘until’) are briefly illustrated. We also formalise concurrent imperative programming constructs with and without Π , and relate the two approaches. Peterson’s mutual exclusion algorithm is used to illustrate reasoning with Π about a concrete programming example. Projection with fairness and non-fairness assumptions are both discussed. This all illustrates an approach to the analysis of such concurrent interleaving finite-state systems using temporal logic formulas with projection constructs to reason about correctness properties. Unlike conventional LTL formulas about concurrency which normally largely focus on global time, properties expressed in LTL combined with Π help to reveal and analyse important differing viewpoints involving global time and the local projected time seen by each individual process. Links between Π and another standard PITL projection operator, both suitable for reasoning about different time granularities, are demonstrated by showing the two operators to be interdefinable. We briefly look at other (mostly interval-based) temporal logics with similar forms of projection, as well as some related applications and industrial standards.