No abstract

Linearizability is the commonly accepted notion of correctness for concurrent data structures. It requires that any execution of the data structure is justified by a linearization -a linear order on operations satisfying the data structure's sequential specification. Proving linearizability is often challenging because an operation's position in the linearization order may depend on future operations. This makes it very difficult to incrementally construct the linearization in a proof. We propose a new proof method that can handle data structures with such futuredependent linearizations. Our key idea is to incrementally construct not a single linear order of operations, but a partial order that describes multiple linearizations satisfying the sequential specification. This allows decisions about the ordering of operations to be delayed, mirroring the behaviour of data structure implementations. We formalise our method as a program logic based on rely-guarantee reasoning, and demonstrate its effectiveness by verifying several challenging data structures: the Herlihy-Wing queue, the TS queue and the Optimistic set. arXiv:1701.05463v4 [cs.PL] 6 Jul 2017 14 Val dequeue() { 15 Val ret := NULL; 16 EventID CAND; 17 do { 18 TS start ts := newTimestamp(); 19 PoolID pid, cand pid := NULL; 20 TS ts, cand ts := ; 21 ThreadID cand tid; 22 * D , , H }. We say that a data structure (D, s 0 ) is linearizable with respect to a set of sequential histories H if H(D, s 0 ) H (Definition 2). LogicWe now formalise our proof method as a Hoare logic based on rely-guarantee [13]. We make this choice to keep presentation simple; our method is general and can be combined with more advanced methods for reasoning about concurrency [22,1,20].

We study the formal semantics of non-volatile memory in the x86-TSO architecture. We show that while the explicit persist operations in the recent model of Raad et al. from POPL'20 only enforce order between writes to the non-volatile memory, it is equivalent, in terms of reachable states, to a model whose explicit persist operations mandate that prior writes are actually written to the non-volatile memory. The latter provides a novel model that is much closer to common developers' understanding of persistency semantics. We further introduce a simpler and stronger sequentially consistent persistency model, develop a sound mapping from this model to x86, and establish a data-race-freedom guarantee providing programmers with a safe programming discipline. Our operational models are accompanied with equivalent declarative formulations, which facilitate our formal arguments, and may prove useful for program verification under x86 persistency.

Transactional memory (TM) facilitates the development of concurrent applications by letting the programmer designate certain code blocks as atomic. Programmers using a TM often would like to access the same data both inside and outside transactions, e.g., to improve performance or to support legacy code. In this case, programmers would ideally like the TM to guarantee strong atomicity , where transactions can be viewed as executing atomically also with respect to non-transactional accesses. Since guaranteeing strong atomicity for arbitrary programs is prohibitively expensive, researchers have suggested guaranteeing it only for certain data-race free (DRF) programs, particularly those that follow the privatization idiom: from some point on, threads agree that a given object can be accessed non-transactionally. Supporting privatization safely in a TM is nontrivial, because this often requires correctly inserting transactional fences , which wait until all active transactions complete. Unfortunately, there is currently no consensus on a single definition of transactional DRF, in particular, because no existing notion of DRF takes into account transactional fences. In this paper we propose such a notion and prove that, if a TM satisfies a certain condition generalizing opacity and a program using it is DRF assuming strong atomicity, then the program indeed has strongly atomic semantics. We show that our DRF notion allows the programmer to use privatization idioms. We also propose a method for proving our generalization of opacity and apply it to the TL2 TM.

Linearizability is a commonly accepted notion of correctness for libraries of concurrent algorithms, and recent years have seen a number of proposals of program logics for proving it. Although these logics differ in technical details, they embody similar reasoning principles. To explicate these principles, we propose a logic for proving linearizability that is generic: it can be instantiated with different means of compositional reasoning about concurrency, such as separation logic or rely-guarantee. To this end, we generalise the Views framework for reasoning about concurrency to handle relations between programs, required for proving linearizability. We present sample instantiations of our generic logic and show that it is powerful enough to handle concurrent algorithms with challenging features, such as helping.

Modern highly-concurrent search data structures, such as search trees, obtain multi-core scalability and performance by having operations traverse the data structure without any synchronization. As a result, however, these algorithms are notoriously difficult to prove linearizable, which requires identifying a point in time in which the traversal's result is correct. The problem is that traversing the data structure as it undergoes modifications leads to complex behaviors, necessitating intricate reasoning about all interleavings of reads by traversals and writes mutating the data structure. In this paper, we present a general proof technique for proving unsynchronized traversals correct in a significantly simpler manner, compared to typical concurrent reasoning and prior proof techniques. Our framework relies only on sequential properties of traversals and on a conceptually simple and widely-applicable condition about the ways an algorithm's writes mutate the data structure. Establishing that a target data structure satisfies our condition requires only simple concurrent reasoning, without considering interactions of writes and reads. This reasoning can be further simplified by using our framework. To demonstrate our technique, we apply it to prove several interesting and challenging concurrent binary search trees: the logical-ordering AVL tree, the Citrus tree, and the full contention-friendly tree. Both the logical-ordering tree and the full contention-friendly tree are beyond the reach of previous approaches targeted at simplifying linearizability proofs.

Speculative execution attacks present an enormous security threat, capable of reading arbitrary program data under malicious speculation, and later exfiltrating that data over microarchitectural covert channels. Since these attacks first rely on being able to read arbitrary data (potential secrets), a conservative approach to defeat all attacks is to delay the execution of instructions that read those secrets, until those instructions become non-speculative. This paper's premise is that it is safe to execute and selectively forward the results of speculative instructions that read secrets, which improves performance, as long as we can prove that the forwarded results do not reach potential covert channels. We propose a comprehensive hardware protection based on this idea, called Speculative Taint Tracking (STT), capable of protecting all speculatively accessed data.Our work addresses two key challenges. First, to safely selectively forward secrets, we must understand what instruction(s) can form covert channels. We provide a comprehensive study of covert channels on speculative microarchitectures, and use this study to develop hardware mechanisms that block each class of channel. Along the way, we find new classes of covert channels related to implicit flow on speculative machines. Second, for performance, it is essential to disable protection on previously protected data, as soon as doing so is safe. We identify that the earliest time is when the instruction(s) producing the protected data become non-speculative, and design a novel microarchitecture for disabling protection at this moment.We provide an extensive formal analysis showing that STT enforces a novel form of non-interference, with respect to all speculatively accessed data. We further evaluate STT on 21 SPEC and 9 PARSEC workloads, and find it adds only 8.5%/14.5% overhead (depending on attack model) relative to an insecure machine, while reducing overhead by 4.7×/18.8× relative to a baseline secure scheme.

We present Abstract Pathfinder, an extension to the Java Pathfinder (JPF) verification tool-set that supports data abstraction to reduce the large data domains of a Java program to small, finite abstract domains, making the program more amenable to verification. We use data abstraction to compute an over-approximation of the original program in such a way that if a (safety) property is true in the abstracted program the property is also true in the original program. Our approach enhances JPF with an abstract interpreter and abstract state-matching mechanisms, together with a library of abstractions from which the user can pick which abstractions to use for a particular application. We discuss the details of our implementation together with some preliminary experiments with analyzing multi-threaded Java programs, where Abstract Pathfinder achieves significant time and memory savings as compared with plain JPF.