Grounding thin-air reads with event structures

Abstract

The key challenge in defining the concurrency semantics of a programming language is how to enable the most efficient compilation to existing hardware architectures, and yet forbid programs from reading thin-air values, i.e., ones that do not appear in the program. At POPL'17, Kang et al. achieved a major breakthrough by introducing the 'promising' semantics that came with results showing that it was a good candidate solution to the problem. Unfortunately, however, the promising semantics is rather complicated, and due to its complexity it contains some flaws and limitations that are very hard to address. In response, we present an alternative solution to this problem based on event structures. We show that it is indeed a solution by establishing the standard results about the semantics (DRF theorems, implementation and optimization correctness) as well as a formal connection to the semantics of Kang et al. Further, we show that it is easier to adapt, by extending the semantics to cover features (such as SC accesses) that are not supported by Kang et al. and to rule out some dubious behaviors admitted by the promising semantics.

Modular Relaxed Dependencies in Weak Memory Concurrency

“…The key insight from these papers is that it is necessary to consider multiple program executions simultaneously. To do this, three of the four [15,25,11] use event structures, while the Promising Semantics [16] is a small-step operational semantics that explores future traces in order to take a step.…”

“…Our tool, MRDer, evaluates litmus tests under the base model, RC11 augmented with MRD, and IMM augmented with MRD. It has been used to check the result of every litmus test in this paper, together with many tests from the literature, including the Java Causality Test cases [7,11,15,16,18,25,26,27].…”

“…At each operational step, the Promising Semantics [16] has to run forwards in a limited local way to validate that promised writes will be reached. The invisible events of Chakraborty et al [11] are used to similar effect.…”

“…It has been a longstanding problem to define the semantics of programming languages with shared memory concurrency in a way that does not allow unwanted behaviours -especially observing thin-air values [8,7] -and that does not forbid compiler optimisations that are important in practice, as is the case with Java and Hotspot [30,29]. Recent attempts [16,11,25,15] have abandoned the style of axiomatic models, which is the de facto paradigm of industrial specification [8,2,6]. Axiomatic models comprise rules that allow or forbid individual program executions.…”

Bridging the gap between programming languages and hardware weak memory models

“…developed RC11, a repaired version of C/C++11, and established (by pen-and-paper proof) the correctness of the suggested compilation schemes to x86-TSO, POWER and ARMv7. Beyond (R)C11, however, there are a number of other proposed higher-level semantics, such as JMM [Manson et al 2005], OCaml [Dolan et al 2018], Promise , LLVM [Chakraborty and Vafeiadis 2017], Linux kernel memory model [Alglave et al 2018], AE-justification [Jeffrey and Riely 2016], Bubbly [Pichon-Pharabod and Sewell 2016], and WeakestMO [Chakraborty and Vafeiadis 2019], for which only a handful of compilation correctness results have been developed.…”