Modularising opacity verification for hybrid transactional memory

Transactional memory (TM) manages thread synchronisation to provide an illusion of atomicity for arbitrary blocks of code. There are various implementations of TM, including hardware (HTM) and software (STM). HTMs provide high performance, but are inherently limited by hardware restrictions; STMs avoid these limitations but suffer from unpredictable performance. To solve these problems, hybrid TM (HyTM) algorithms have been introduced which provide reliable software fallback mechanisms for hardware transactions. The key safety property for TM is opacity, however a naive combination of an opaque STM and an opaque HTM does not necessarily result in an opaque HyTM. Therefore, HyTM algorithms must be specially designed to satisfy opacity. In this paper we introduce a modular method for verifying opacity of HyTM implementations. Our method provides conditions under which opacity proofs of HTM and STM components can be combined into a single proof of an overall hybrid algorithm. The proof method has been fully mechanised in Isabelle, and used to verify a novel hybrid version of a transactional mutex lock

Modularising Verification Of Durable Opacity

Non-volatile memory (NVM), also known as persistent memory, is an emerging paradigm for memory that preserves its contents even after power loss. NVM is widely expected to become ubiquitous, and hardware architectures are already providing support for NVM programming. This has stimulated interest in the design of novel concepts ensuring correctness of concurrent programming abstractions in the face of persistency and in the development of associated verification approaches. Software transactional memory (STM) is a key programming abstraction that supports concurrent access to shared state. In a fashion similar to linearizability as the correctness condition for concurrent data structures, there is an established notion of correctness for STMs known as opacity. We have recently proposed durable opacity as the natural extension of opacity to a setting with non-volatile memory. Together with this novel correctness condition, we designed a verification technique based on refinement. In this paper, we extend this work in two directions. First, we develop a durably opaque version of NOrec (no ownership records), an existing STM algorithm proven to be opaque. Second, we modularise our existing verification approach by separating the proof of durability of memory accesses from the proof of opacity. For NOrec, this allows us to re-use an existing opacity proof and complement it with a proof of the durability of accesses to shared state

Defining and Verifying Durable Opacity: Correctness for Persistent Software Transactional Memory

Non-volatile memory (NVM), aka persistent memory, is a new paradigm for memory that preserves its contents even after power loss. The expected ubiquity of NVM has stimulated interest in the design of novel concepts ensuring correctness of concurrent programming abstractions in the face of persistency. So far, this has lead to the design of a number of persistent concurrent data structures, built to satisfy an associated notion of correctness: durable linearizability. In this paper, we transfer the principle of durable concurrent correctness to the area of software transactional memory (STM). Software transactional memory algorithms allow for concurrent access to shared state. Like linearizability for concurrent data structures, opacity is the established notion of correctness for STMs. First, we provide a novel definition of durable opacity extending opacity to handle crashes and recovery in the context of NVM. Second, we develop a durably opaque version of an existing STM algorithm, namely the Transactional Mutex Lock (TML). Third, we design a proof technique for durable opacity based on refinement between TML and an operational characterisation of durable opacity by adapting the TMS2 specification. Finally, we apply this proof technique to show that the durable version of TML is indeed durably opaque. The correctness proof is mechanized within Isabelle.Comment: This is the full version of the paper that is to appear in FORTE 2020 (https://www.discotec.org/2020/forte