A criterion for atomicity

Abstract

Abstract Most proof methods for reasoning about concurrent programs are based upon the interleaving semantics of concurrent computation: a concurrent program is executed in a stepwise fashion, with only one enabled action being executed at each step. Interleaving semantics, in effect, requires that a concurrent program be executed as a nondeterministic sequential program. This is clearly an abstraction of the way in which concurrent programs are actually executed. To ensure that this is a reasonable abstraction, interleaving semantics should only be used to reason about programs with “simple” actions; we call such programs “atomic”. In this paper, we formally characterise the class of atomic programs. We adopt the criterion that a program is atomic if it can be implemented in a wait-free, serialisable manner by a primitive program. A program is primitive if each of its actions has at most one occurrence of a shared bit, and each shared bit is read by at most one process and written by at most one process. It follows from our results that the traditionally accepted atomicity criterion, which allows each action to have at most one occurrence of a shared variable, can be relaxed, allowing programs to have more powerful actions. For example, according to our criterion, an action can read any finite number of shared variables, provided it writes no shared variable.