Concurrent Data Structures Linked in Time

Arguments about correctness of a concurrent data structure are typically carried out by using the notion of linearizability and specifying the linearization points of the data structure's procedures. Such arguments are often cumbersome as the linearization points' position in time can be dynamic (depend on the interference, run-time values and events from the past, or even future), non-local (appear in procedures other than the one considered), and whose position in the execution trace may only be determined after the considered procedure has already terminated. In this paper we propose a new method, based on a separation-style logic, for reasoning about concurrent objects with such linearization points. We embrace the dynamic nature of linearization points, and encode it as part of the data structure's auxiliary state, so that it can be dynamically modified in place by auxiliary code, as needed when some appropriate run-time event occurs. We name the idea linking-in-time, because it reduces temporal reasoning to spatial reasoning. For example, modifying a temporal position of a linearization point can be modeled similarly to a pointer update in separation logic. Furthermore, the auxiliary state provides a convenient way to concisely express the properties essential for reasoning about clients of such concurrent objects. We illustrate the method by verifying (mechanically in Coq) an intricate optimal snapshot algorithm due to Jayanti, as well as some clients

Reconfigurable Lattice Agreement and Applications

Reconfiguration is one of the central mechanisms in distributed systems. Due to failures and connectivity disruptions, the very set of service replicas (or servers) and their roles in the computation may have to be reconfigured over time. To provide the desired level of consistency and availability to applications running on top of these servers, the clients of the service should be able to reach some form of agreement on the system configuration. We observe that this agreement is naturally captured via a lattice partial order on the system states. We propose an asynchronous implementation of reconfigurable lattice agreement that implies elegant reconfigurable versions of a large class of lattice abstract data types, such as max-registers and conflict detectors, as well as popular distributed programming abstractions, such as atomic snapshot and commit-adopt

Modelling concurrent objects running on the TSO and ARMv8 memory models

Hardware weak memory models, such as TSO and ARM, are used to increase the performance of concurrent programs by allowing program instructions to be executed on the hardware in a different order to that specified by the software. This places a challenge on the verification of concurrent objects used in these programs since the variations in the executions need to be considered. Many approaches exist for verifying concurrent objects along with associated tool support. In particular, we focus on a thread-local approach to checking linearizability, the standard correctness condition for concurrent objects, using a model checker. This approach, like most others, does not support weak memory models. In order to reuse this existing approach, therefore, we show how to use the semantics of a weak memory model to directly derive a transition system of concurrent objects running under it. We do this for both TSO and the latest version of ARM, ARMv8. Since there is a straightforward implementation of TSO, we reflect this in our transition system which includes a buffer of writes to memory mirroring the store buffer of TSO. We illustrate linearizability checking using model checking on a transition system generated by this approach. The implementation of the significantly more complex ARMv8 architecture is less obvious. We derive our transition system in this case from an exisiting operational semantics that is consistent with the results of thousands of litmus test run on ARM hardware