No abstract

While relaxed memory consistency models contribute optimizations of compilers on multicore CPUs and shared memory distributed programming languages, their relaxedness makes it difficult to write programs correctly. To address this problem, the authors proposed a general model checking framework and implemented a prototype tool McSPIN, which can take a memory consistent model as an input, as well as a program and a property to be checked, in their previous works. However, one big problem of McSPIN was that it was prone to suffer from the state explosion problem, and difficult to be applied to programs other than small example programs. In this paper, we propose optimization approaches for McSPIN to largely reduce the number of state transitions to be explored during model checking so that it can be applied to larger programs. In addition, we actually implemented the optimizations to McSPIN, and this paper gives several experimental results with the optimized McSPIN to show effectiveness of the proposed optimization approaches.

No abstract

Abstract:Relaxed memory consistency models specify effects of executions of statements among threads, which may or may not be reordered. Such reorderings may cross loop iterations. To the best of our knowledge, however, there exists no concurrent program logic which explicitly handles the reorderings across loop iterations. This paper provides concurrent program logic for relaxed memory consistency models that can represent, for example, total store ordering, partial store ordering, relaxed memory ordering, and acquire and release consistency. There are two novel aspects to our approach. First, we translate a concurrent program into a family of directed acyclic graphs with finite nodes and transitive edges called program graphs according to a memory consistency model that we adopt. These represent dependencies among statements which represent reorderings of not only statements but also visibility of their effects. Second, we introduce auxiliary variables that temporarily buffer the effects of write operations on shared memory, and explicitly describe the reflections of the buffered effects to shared memory. Specifically, we define a small-step operational semantics for the program graphs with the introduced auxiliary variables, then define sound and relatively complete logic to the semantics.

Modern concurrent copying garbage collection (GC), in particular, real-time GC, uses fine-grained synchronizations with a mutator, which is the application program that mutates memory, when it moves objects in its copy phase. It resolves a data race using a concurrent copying protocol, which is implemented as interactions between the collector threads and the read and write barriers that the mutator threads execute. The behavioral effects of the concurrent copying protocol rely on the memory model of the CPUs and the programming languages in which the GC is implemented. It is difficult, however, to formally investigate the behavioral properties of concurrent copying protocols against various memory models. To address this problem, we studied the feasibility of the bounded model checking of concurrent copying protocols with memory models. We investigated a correctness-related behavioral property of copying protocols of various concurrent copying GC algorithms, including real-time GC Stopless, Clover, Chicken, Staccato, and Schism against six memory models, total store ordering (TSO), partial store ordering (PSO), relaxed memory ordering (RMO), and their variants, in addition to sequential consistency (SC) using bounded model checking. For each combination of a protocol and memory model, we conducted model checking with a model of a mutator. In this wide range of case studies, we found faults in two GC algorithms, one of which is relevant to the memory model. We fixed these faults with the great help of counterexamples. We also modified some protocols so that they work under some memory models weaker than those for which the original protocols were designed, and checked them using model checking. We believe that bounded model checking is a feasible approach to investigate behavioral properties of concurrent copying protocols under weak memory models. CCS Concepts: • Software and its engineering → Garbage collection; Model checking;