How Hard is Weak-Memory Testing?

Weak-memory models are standard formal specifications of concurrency across hardware, programming languages, and distributed systems. A fundamental computational problem is consistency testing: is the observed execution of a concurrent program in alignment with the specification of the underlying system? The problem has been studied extensively across Sequential Consistency (SC) and weak memory, and proven to be NP-complete when some aspect of the input (e.g., number of threads/memory locations) is unbounded. This unboundedness has left a natural question open: are there efficient parameterized algorithms for testing? The main contribution of this paper is a deep hardness result for consistency testing under many popular weak-memory models: the problem remains NP-complete even in its bounded setting, where candidate executions contain a bounded number of threads, memory locations, and values. This hardness spreads across several Release-Acquire variants of C11, a popular variant of its Relaxed fragment, popular Causal Consistency models, and the POWER architecture. To our knowledge, this is the first result that fully exposes the hardness of weak-memory testing and proves that the problem admits no parameterization under standard input parameters. It also yields a computational separation of these models from SC, x86-TSO, PSO, and Relaxed, for which bounded consistency testing is either known (for SC), or shown here (for the rest), to be in polynomial time

Overcoming Memory Weakness with Unified Fairness

We consider the verification of liveness properties for concurrent programs running on weak memory models. To that end, we identify notions of fairness that preclude demonic non-determinism, are motivated by practical observations, and are amenable to algorithmic techniques. We provide both logical and stochastic definitions of our fairness notions and prove that they are equivalent in the context of liveness verification. In particular, we show that our fairness allows us to reduce the liveness problem (repeated control state reachability) to the problem of simple control state reachability. We show that this is a general phenomenon by developing a uniform framework which serves as the formal foundation of our fairness definition and can be instantiated to a wide landscape of memory models. These models include SC, TSO, PSO, (Strong/Weak) Release-Acquire, Strong Coherence, FIFO-consistency, and RMO.Comment: 32 pages. To appear in Proc. 35th International Conference on Computer Aided Verification (CAV) 202

The Decidability of Verification under Promising 2.0

In PLDI'20, Lee et al. introduced the \emph{promising } semantics PS 2.0 of the C++ concurrency that captures most of the common program transformations while satisfying the DRF guarantee. The reachability problem for finite-state programs under PS 2.0 with only release-acquire accesses is already known to be undecidable. Therefore, we address, in this paper, the reachability problem for programs running under PS 2.0 with relaxed accesses together with promises. We show that this problem is undecidable even in the case where the input program has finite state. Given this undecidability result, we consider the fragment of PS 2.0 with only relaxed accesses allowing bounded number of promises. We show that under this restriction, the reachability is decidable, albeit very expensive: it is non-primitive recursive. Given this high complexity with bounded number of promises and the undecidability result for the RA fragment of PS 2.0, we consider a bounded version of the reachability problem. To this end, we bound both the number of promises and the "view-switches", i.e, the number of times the processes may switch their local views of the global memory. We provide a code-to-code translation from an input program under PS 2.0, with relaxed and release-acquire memory accesses along with promises, to a program under SC. This leads to a reduction of the bounded reachability problem under PS 2.0 to the bounded context-switching problem under SC. We have implemented a prototype tool and tested it on a set of benchmarks, demonstrating that many bugs in programs can be found using a small bound

Correct-by-Construction Reinforcement Learning of Cardiac Pacemakers from Duration Calculus Requirements

As the complexity of pacemaker devices continues to grow, the importance of capturing its functional correctness requirement formally cannot be overestimated. The pacemaker system specification document by \emph{Boston Scientific} provides a widely accepted set of specifications for pacemakers. As these specifications are written in a natural language, they are not amenable for automated verification, synthesis, or reinforcement learning of pacemaker systems. This paper presents a formalization of these requirements for a dual-chamber pacemaker in \emph{duration calculus} (DC), a highly expressive real-time specification language. The proposed formalization allows us to automatically translate pacemaker requirements into executable specifications as stopwatch automata, which can be used to enable simulation, monitoring, validation, verification and automatic synthesis of pacemaker systems. The cyclic nature of the pacemaker-heart closed-loop system results in DC requirements that compile to a decidable subclass of stopwatch automata. We present shield reinforcement learning (shield RL), a shield synthesis based reinforcement learning algorithm, by automatically constructing safety envelopes from DC specifications

imaging

149 Multi slice wide band SSFP CINE for routine 3 T cardia

Improved coronary MR angiography using wideband steady state free precession at 3 Tesla with sub-millimeter resolution

Purpose: To suppress off-resonance artifacts in coronary artery imaging at 3 Tesla (T), and therefore improve spatial resolution. Materials and Methods: Wideband steady state free precession (SSFP) sequences use an oscillating steady state to reduce banding artifacts. Coronary artery images were obtained at 3T using three-dimensional navigated gradient echo, balanced SSFP, and wideband SSFP sequences. Results: The highest in-plane resolution of left coronary artery images was 0.68 mm in the frequency-encoding direction. Wideband SSFP produced an average SNR efficiency of 70% relative to conventional balanced SSFP and suppressed off-resonance artifacts. Conclusion: Wideband SSFP was found to be a promising approach for obtaining noncontrast, high-resolution coronary artery images at 3 Tesla with reliable image quality