The Reads-From Equivalence for the TSO and PSO Memory Models

The verification of concurrent programs remains an open challenge due to the non-determinism in inter-process communication. Instead of exploring concrete executions, stateless model-checking (SMC) techniques partition the execution space into equivalence classes, and explore each class as opposed to each execution. For the relaxed memory models of TSO and PSO (total/partial store order), the standard equivalence has been Shasha-Snir traces, seen as an extension of the classic Mazurkiewicz equivalence from SC (sequential consistency) to TSO and PSO. The reads-from (RF) equivalence was recently shown to be coarser than the Mazurkiewicz equivalence, leading to impressive scalability improvements for SMC under SC. The generalization of RF to TSO and PSO requires to overcome two challenges, namely, verifying execution consistency and SMC algorithm. We address these two fundamental problems in this work. Our first set of contributions is on the problem of verifying TSO- and PSO-consistent executions given a reads-from map, VTSO-rf and VPSO-rf, respectively. The problem has been heavily studied under SC due to its numerous applications, but little is known for TSO and PSO. For an execution of $n$ events over $k$ threads and $d$ variables, we establish novel bounds that scale as $n^{k+1}$ for TSO and as $n^{k+1}\cdot \min(n^{k^2}, 2^{k\cdot d})$ for PSO. Our second contribution is an algorithm for SMC under TSO and PSO using the RF equivalence. Our algorithm is exploration-optimal, in the sense that it is guaranteed to explore each class of the RF partitioning exactly once, and spends polynomial time per class when $k$ is bounded. Our experimental evaluation shows that the RF equivalence is often exponentially coarser than Shasha-Snir traces, and our SMC algorithm scales much better than state-of-the-art tools based on Shasha-Snir traces

IST Austria Thesis

The design and verification of concurrent systems remains an open challenge due to the non-determinism that arises from the inter-process communication. In particular, concurrent programs are notoriously difficult both to be written correctly and to be analyzed formally, as complex thread interaction has to be accounted for. The difficulties are further exacerbated when concurrent programs get executed on modern-day hardware, which contains various buffering and caching mechanisms for efficiency reasons. This causes further subtle non-determinism, which can often produce very unintuitive behavior of the concurrent programs. Model checking is at the forefront of tackling the verification problem, where the task is to decide, given as input a concurrent system and a desired property, whether the system satisfies the property. The inherent state-space explosion problem in model checking of concurrent systems causes naïve explicit methods not to scale, thus more inventive methods are required. One such method is stateless model checking (SMC), which explores in memory-efficient manner the program executions rather than the states of the program. State-of-the-art SMC is typically coupled with partial order reduction (POR) techniques, which argue that certain executions provably produce identical system behavior, thus limiting the amount of executions one needs to explore in order to cover all possible behaviors. Another method to tackle the state-space explosion is symbolic model checking, where the considered techniques operate on a succinct implicit representation of the input system rather than explicitly accessing the system. In this thesis we present new techniques for verification of concurrent systems. We present several novel POR methods for SMC of concurrent programs under various models of semantics, some of which account for write-buffering mechanisms. Additionally, we present novel algorithms for symbolic model checking of finite-state concurrent systems, where the desired property of the systems is to ensure a formally defined notion of fairness

Concurrency with Weak Memory Models (Dagstuhl Seminar 16471)

This report documents the program and the outcomes of Dagstuhl Seminar 16471 "Concurrency with Weak Memory Models: Semantics, Languages, Compilation, Verification, Static Analysis, and Synthesis". The aim of the seminar was to bring together people from various horizons, including theoreticians and verification practitioners as well as hardware vendors, in order to set up a long-term research program to design formal methods and develop tools ensuring the correctness of concurrent programs on modern multi-processor architectures