Verifying Visibility-Based Weak Consistency

TL;DR

This paper presents a methodology for verifying that concurrent objects in multithreaded programs adhere to weak consistency models by using annotations and automated proofs, ensuring correctness despite relaxed visibility guarantees.

Contribution

It introduces a formal, annotation-based approach for proving weak-consistency adherence in concurrent object implementations using simulation proofs and automated theorem proving.

Findings

Methodology effectively verifies weak-consistency in Java concurrent objects.

Annotations can be automated by tracking memory location writers.

Formal proofs ensure correctness despite relaxed visibility assumptions.

Abstract

Multithreaded programs generally leverage efficient and thread-safe concurrent objects like sets, key-value maps, and queues. While some concurrent-object operations are designed to behave atomically, each witnessing the atomic effects of predecessors in a linearization order, others forego such strong consistency to avoid complex control and synchronization bottlenecks. For example, contains (value) methods of key-value maps may iterate through key-value entries without blocking concurrent updates, to avoid unwanted performance bottlenecks, and consequently overlook the effects of some linearization-order predecessors. While such weakly-consistent operations may not be atomic, they still offer guarantees, e.g., only observing values that have been present. In this work we develop a methodology for proving that concurrent object implementations adhere to weak-consistency specifications. In particular, we consider (forward) simulation-based proofs of implementations against relaxed-visibility specifications, which allow designated operations to overlook some of their linearization-order predecessors, i.e., behaving as if they never occurred. Besides annotating implementation code to identify linearization points, i.e., points at which operations' logical effects occur, we also annotate code to identify visible operations, i.e., operations whose effects are observed; in practice this annotation can be done automatically by tracking the writers to each accessed memory location. We formalize our methodology over a general notion of transition systems, agnostic to any particular programming language or memory model, and demonstrate its application, using automated theorem provers, by verifying models of Java concurrent object implementations.