Conflict-Freedom as a Progress Condition

TL;DR

This paper introduces conflict-freedom, a progress condition that generalizes obstruction-freedom by allowing concurrent conflicting operations without hindering progress, especially for objects with semantic commutativity.

Contribution

It formalizes conflict-freedom, proves that all sequential objects have conflict-free linearizable implementations, and presents a novel conflict-free universal construction.

Findings

Every sequential object has a conflict-free linearizable implementation.

Conflict-freedom allows progress despite conflicting operations if they are incomplete.

A new conflict-free universal construction based on a generalized commit-adopt object.

Abstract

An obstruction-free implementation guarantees progress to every operation that is given enough time to take steps in isolation. But, as we show in this paper, the mere presence of concurrent operations alone does not have to prevent progress; only incomplete conflicting (non-commuting) operations may do so. This progress condition, that we call conflict-freedom, is a natural generalization of obstruction-freedom that promises efficient implementations for objects exhibiting semantic commutativity. We show that, as with obstruction-freedom, every sequential object has a read-write conflict-free linearizable implementation. Our conflict-free universal construction is based on a novel generalization of the instrumental commit-adopt object, interesting in its own right.