Pending Conflicts Make Progress Impossible

2026-02-03

AI Summary

This study investigates concurrency progress conditions for linearizable shared objects that exploit commutativity-awareness in the asynchronous read-write shared-memory model. Addressing limitations of existing progress guarantees, the paper introduces a novel condition termed *conflict-obstruction-freedom*, which ensures that a process can complete its operation even when contending only with commutative operations. Leveraging formal tools from concurrent computation theory and linearizability semantics, the work provides the first rigorous formulation of this condition and proves that a universal construction satisfying it is impossible in the asynchronous read-write model. This impossibility result demonstrates that synchronization overhead remains unavoidable even when contention arises solely from conflicting—yet potentially commutative—operations, thereby establishing a fundamental limitation on progress guarantees in commutativity-aware concurrent systems.

Technology Category

• Constraint Satisfaction and Optimization: Satisfiability Modulo Theories
• Knowledge Representation and Reasoning: Computational Complexity of Reasoning
• Reasoning under Uncertainty: Other Foundations of Reasoning under Uncertainty

Application Category

• Security and Privacy: Data transparency and provenance
• Graph Algorithms and Modeling for the Web: Algorithms and analysis for incomplete, noisy, or partially observed Web-related graphs
• Systems and Infrastructure for Web, Mobile and WoT: Web performance, measurement, and characterization

Abstract

In this work, we study progress conditions for commutativity-aware, linearizable implementations of shared objects. Motivated by the observation that commuting operations can be executed in parallel, we introduce conflict-obstruction-freedom: a process is guaranteed to complete its operation if it runs for long enough without encountering step contention with conflicting (non-commuting) operations. This condition generalizes obstruction-freedom and wait-freedom by allowing progress as long as step contention is only induced by commuting operations. We prove that conflict-obstruction-free universal constructions are impossible to implement in the asynchronous read-write shared memory model. This result exposes a fundamental limitation of conflict-aware universal constructions: the mere invocation of conflicting operations imposes a synchronization cost. Progress requires eventual resolution of pending conflicts.

Problem

• conflict-obstruction-freedom
• commutativity
• linearizable
• universal construction
• asynchronous shared memory

Innovation

• conflict-obstruction-freedom
• commutativity
• linearizability
• universal construction
• asynchronous shared memory