Imperial College London

DrAzaleaRaad

Faculty of EngineeringDepartment of Computing

Lecturer
 
 
 
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Contact

 

+44 (0)20 7594 8271azalea.raad Website

 
 
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Location

 

Huxley BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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25 results found

Cho K, Lee S-H, Raad A, Kang Jet al., 2021, Revamping hardware persistency models: view-based and axiomatic persistency models for Intel-x86 and Armv8, PLDI '21: 42nd ACM SIGPLAN International Conference on Programming Language Design and Implementation, Publisher: ACM, Pages: 16-31

Non-volatile memory (NVM) is a cutting-edge storage technology that promises the performance of DRAM with the durability of SSD. Recent work has proposed several persistency models for mainstream architectures such as Intel-x86 and Armv8, describing the order in which writes are propagated to NVM. However, these models have several limitations; most notably, they either lack operational models or do not support persistent synchronization patterns.We close this gap by revamping the existing persistency models. First, inspired by the recent work on promising semantics, we propose a unified operational style for describing persistency using views, and develop view-based operational persistency models for Intel-x86 and Armv8, thus presenting the first operational model for Armv8 persistency. Next, we propose a unified axiomatic style for describing hardware persistency, allowing us to recast and repair the existing axiomatic models of Intel-x86 and Armv8 persistency. We prove that our axiomatic models are equivalent to the authoritative semantics reviewed by Intel and Arm engineers. We further prove that each axiomatic hardware persistency model is equivalent to its operational counterpart. Finally, we develop a persistent model checking algorithm and tool, and use it to verify several representative examples.

Conference paper

Kokologiannakis M, Kaysin I, Raad A, Vafeiadis Vet al., 2021, PerSeVerE: persistency semantics for verification under ext4., Proceedings of the ACM on Programming Languages, Vol: 5, Pages: 1-29, ISSN: 2475-1421

Although ubiquitous, modern filesystems have rather complex behaviours that are hardly understood by programmers and lead to severe software bugs such as data corruption. As a first step to ensure correctness of software performing file I/O, we formalize the semantics of the Linux ext4 filesystem, which we integrate with the weak memory consistency semantics of C/C++. We further develop an effective model checking approach for verifying programs that use the filesystem. In doing so, we discover and report bugs in commonly-used text editors such as vim, emacs and nano.

Journal article

Raad A, Lahav O, Vafeiadis V, 2020, Persistent owicki-gries reasoning: a program logic for reasoning about persistent programs on Intel-x86, Proceedings of the ACM on Programming Languages, Vol: 4, Pages: 1-28, ISSN: 2475-1421

The advent of non-volatile memory (NVM) technologies is expected to transform how software systems are structured fundamentally, making the task of correct programming significantly harder. This is because ensuring that memory stores persist in the correct order is challenging, and requires low-level programming to flush the cache at appropriate points. This has in turn resulted in a noticeable verification gap.To address this, we study the verification of NVM programs, and present Persistent Owicki-Gries (POG), the first program logic for reasoning about such programs. We prove the soundness of POG over the recent Intel-x86 model, which formalises the out-of-order persistence of memory stores and the semantics of the Intel cache line flush instructions. We then use POG to verify several programs that interact with NVM.

Journal article

Xiong S, Cerone A, Raad A, Gardner Pet al., 2020, Data Consistency in Transactional Storage Systems: a Centralised Approach, 41st ACM SIGPLAN International Conference on Programming Language Design and Implementation (PLDI '20)

Conference paper

Raad A, Wickerson J, Neiger G, Vafeiadis Vet al., 2020, Persistency semantics of the Intel-x86 architecture, ACM Principles of Programming Languages, Publisher: Association for Computing Machinery (ACM), Pages: 11:1-11:31, ISSN: 2475-1421

Emerging non-volatile memory (NVM) technologies promise the durability of disks with the performance of RAM. To describe the persistency guarantees of NVM, several memory persistency models have been proposed in the literature. However, the persistency semantics of the ubiquitous x86 architecture remains unexplored to date. To close this gap, we develop the Px86 (‘persistent x86’) model, formalising the persistency semantics of Intel-x86 for the first time. We formulate Px86 both operationally and declaratively, and prove that the two characterisations are equivalent. To demonstrate the application of Px86, we develop two persistent libraries over Px86: a persistent transactional library, and a persistent variant of the Michael–Scott queue. Finally, we encode our declarative Px86 model in Alloy and use it to generate persistency litmus tests automatically.

Conference paper

Raad A, Berdine J, Dang H-H, Dreyer D, O'Hearn PW, Villard Jet al., 2020, Local Reasoning About the Presence of Bugs: Incorrectness Separation Logic., Publisher: Springer, Pages: 225-252

Conference paper

Raad A, Lahav O, Vafeiadis V, 2020, Persistent Owicki-Gries reasoning: a program logic for reasoning about persistent programs on Intel-x86., Proc. ACM Program. Lang., Vol: 4, Pages: 151:1-151:1

Journal article

Raad A, Wickerson J, Vafeiadis V, 2019, Weak persistency semantics from the ground up: formalising the persistency semantics of ARMv8 and transactional models, OOPSLA 2019, Publisher: Association for Computing Machinery (ACM), ISSN: 2475-1421

Emerging non-volatile memory (NVM) technologies promise the durability of disks with the performance of volatile memory (RAM). To describe the persistency guarantees of NVM, several memory persistency models have been proposed in the literature. However, the formal persistency semantics of mainstream hardware is unexplored to date. To close this gap, we present a formal declarative framework for describing concurrency models in the NVM context, and then develop the PARMv8 persistency model as an instance of our framework, formalising the persistency semantics of the ARMv8 architecture for the first time. To facilitate correct persistent programming, we study transactions as a simple abstraction for concurrency and persistency control. We thus develop the PSER (persistent serialisability) persistency model, formalising transactional semantics in the NVM context for the first time, and demonstrate that PSER correctly compiles to PARMv8. This then enables programmers to write correct, concurrent and persistent programs, without having to understand the low-level architecture-specific persistency semantics of the underlying hardware.

Conference paper

Xiong S, Cerone A, Raad A, Gardner Pet al., 2019, Data consistency in transactional storage systems: a centralised approach., Publisher: arXiv

We introduce an interleaving operational semantics for describing the client-observable behaviour of atomic transactions on distributed key-value stores. Our semantics builds on abstract states comprising centralised, global key-value stores and partial client views. We provide operational definitions of consistency models for our key-value stores which are shown to be equivalent to the well-known declarative definitions of consistency model for execution graphs. We explore two immediate applications of our semantics: specific protocols of geo-replicated databases (e.g. COPS) and partitioned databases (e.g. Clock-SI) can be shown to be correct for a specific consistency model by embedding them in our centralised semantics; programs can be directly shown to have invariant properties such as robustness results against a weak consistency model.

Working paper

Kokologiannakis M, Raad A, Vafeiadis V, 2019, Effective lock handling in stateless model checking., Proceedings of the ACM on Programming Languages, Vol: 3, Pages: 173:1-173:26, ISSN: 2475-1421

Stateless Model Checking (SMC) is a verification technique for concurrent programs that checks for safety violations by exploring all possible thread interleavings. SMC is usually coupled with Partial Order Reduction (POR), which exploits the independence of instructions to avoid redundant explorations when an equivalent one has already been considered. While effective POR techniques have been developed for many different memory models, they are only able to exploit independence at the instruction level, which makes them unsuitable for programs with coarse-grained synchronization mechanisms such as locks.We present a lock-aware POR algorithm, LAPOR, that exploits independence at both instruction and critical section levels. This enables LAPOR to explore exponentially fewer interleavings than the state-of-the-art techniques for programs that use locks conservatively. Our algorithm is sound, complete, and optimal, and can be used for verifying programs under several different memory models. We implement LAPOR in a tool and show that it can be exponentially faster than the state-of-the-art model checkers.

Journal article

Raad A, Wickerson J, Vafeiadis V, 2019, Weak persistency semantics from the ground up: formalising the persistency semantics of ARMv8 and transactional models, Proceedings of the ACM on Programming Languages, ISSN: 2475-1421

Journal article

Kokologiannakis M, Raad A, Vafeiadis V, 2019, Model checking for weakly consistent libraries., PLDI19, Publisher: Association for Computing Machinery (ACM), Pages: 96-110

Conference paper

Raad A, Lahav O, Vafeiadis V, 2019, On the semantics of snapshot isolation., 20th International Conference, VMCAI, Publisher: Springer, Pages: 1-23

Snapshot isolation (SI) is a standard transactional consistency model used in databases, distributed systems and software transactional memory (STM). Its semantics is formally defined both declaratively as an acyclicity axiom, and operationally as a concurrent algorithm with memory bearing timestamps.We develop two simpler equivalent operational definitions of SI as lock-based reference implementations that do not use timestamps. Our first locking implementation is prescient in that requires a priori knowledge of the data accessed by a transaction and carries out transactional writes eagerly (in-place). Our second implementation is non-prescient and performs transactional writes lazily by recording them in a local log and propagating them to memory at commit time. Whilst our first implementation is simpler and may be better suited for developing a program logic for SI transactions, our second implementation is more practical due to its non-prescience. We show that both implementations are sound and complete against the declarative SI specification and thus yield equivalent operational definitions for SI.We further consider, for the first time formally, the use of SI in a context with racy non-transactional accesses, as can arise in STM implementations of SI. We introduce robust snapshot isolation (RSI), an adaptation of SI with similar semantics and guarantees in this mixed setting. We present a declarative specification of RSI as an acyclicity axiom and analogously develop two operational models as lock-based reference implementations (one eager, one lazy). We show that these operational models are both sound and complete against the declarative RSI model.

Conference paper

Raad A, Doko M, Rozic L, Lahav O, Vafeiadis Vet al., 2019, On library correctness under weak memory consistency: specifying and verifying concurrent libraries under declarative consistency models, ACM Principles of Programming Languages, Publisher: Association for Computing Machinery (ACM), Pages: 68: 1-68: 31, ISSN: 2475-1421

Concurrent libraries are the building blocks for concurrency. They encompass a range of abstractions (locks, exchangers, stacks, queues, sets) built in a layered fashion: more advanced libraries are built out of simpler ones. While there has been a lot of work on verifying such libraries in a sequentially consistent (SC) environment, little is known about how to specify and verify them under weak memory consistency (WMC).We propose a general declarative framework that allows us to specify concurrent libraries declaratively, and to verify library implementations against their specifications compositionally. Our framework is sufficient to encode standard models such as SC, (R)C11 and TSO. Additionally, we specify several concurrent libraries, including mutual exclusion locks, reader-writer locks, exchangers, queues, stacks and sets. We then use our framework to verify multiple weakly consistent implementations of locks, exchangers, queues and stacks.

Conference paper

Cook M, Raad A, 2019, Hyperstate Space Graphs for Automated Game Analysis., Publisher: IEEE, Pages: 1-8

Conference paper

Raad A, Vafeiadis V, 2018, Persistence semantics for weak memory: integrating epoch persistency with the TSO memory model., ACM Object-Oriented Programming, Systems, Languages & Applications (OOPSLA18), Publisher: Association for Computing Machinery (ACM), Pages: 137:1-137:27, ISSN: 2475-1421

Emerging non-volatile memory (NVM) technologies promise the durability of disks with the performance of volatile memory (RAM). To describe the persistency guarantees of NVM, several memory persistency models have been proposed in the literature. However, the formal semantics of such persistency models in the context of existing mainstream hardware has been unexplored to date. To close this gap, we integrate the buffered epoch persistency model with the 'total-store-order' (TSO) memory model of the x86 and SPARC architectures. We thus develop the PTSO ('persistent' TSO) model and formalise its semantics both operationally and declaratively. We demonstrate that the two characterisations of PTSO are equivalent. We then formulate the notion of persistent linearisability to establish the correctness of library implementations in the context of persistent memory. To showcase our formalism, we develop two persistent implementations of a queue library, and apply persistent linearisability to show their correctness.

Conference paper

Raad A, Lahav O, Vafeiadis V, 2018, On Parallel Snapshot Isolation and Release/Acquire Consistency., Publisher: Springer, Pages: 940-967

Conference paper

Cook M, Colton S, Raad A, 2018, Inferring Design Constraints From Game Ruleset Analysis., Publisher: IEEE, Pages: 1-8

Conference paper

Raad A, Hobor A, Villard J, Gardner Pet al., 2016, Verifying concurrent graph algorithms, Asian Symposium on Programming Languages and Systems, Publisher: Springer Verlag, Pages: 314-334, ISSN: 0302-9743

We show how to verify four challenging concurrent fine-grained graph-manipulating algorithms, including graph copy, a speculatively parallel Dijkstra, graph marking and spanning tree. We develop a reasoning method for such algorithms that dynamically tracks the contributions and responsibilities of each thread operating on a graph, even in cases of arbitrary recursive thread creation. We demonstrate how to use a logic without abstraction ( Open image in new window ) to carry out abstract reasoning in the style of iCAP, by building the abstraction into the proof structure rather than incorporating it into the semantic model of the logic.

Conference paper

Raad A, Fragoso Santos J, Gardner P, 2016, DOM: Specification and Client Reasoning, Asian Symposium on Programming Languages and Systems, Publisher: Springer Verlag, Pages: 401-422, ISSN: 0302-9743

We present an axiomatic specification of a key fragment of DOM using structural separation logic. This specification allows us to develop modular reasoning about client programs that call the DOM.

Conference paper

Gardner PA, Raad A, Villard J, 2015, CoLoSL: Concurrent Local Subjective Logic, 24th European Symposium on Programming, ESOP 2015, Publisher: Springer, Pages: 710-735, ISSN: 0302-9743

Conference paper

Gardner P, Raad A, Wheelhouse M, Wright Aet al., 2014, Abstract Local Reasoning for Concurrent Libraries: Mind the Gap, Mathematical Foundations of Programming Semantics Thirtieth Conference, MFPS 2014

Conference paper

Cook M, Colton S, Raad A, Gow Jet al., 2013, Mechanic Miner: Reflection-Driven Game Mechanic Discovery and Level Design., Publisher: Springer, Pages: 284-293

Conference paper

Colton S, Cook M, Raad A, 2011, Ludic Considerations of Tablet-Based Evo-Art, Conference on EvoApplications 2011: EvoCOMPLEX, EvoGAMES, EvoIASP, EvoINTELLIGENCE, EvoNUM, AND EvoSTOC, Publisher: SPRINGER-VERLAG BERLIN, Pages: 223-233, ISSN: 0302-9743

Conference paper

Drossopoulou S, Raad A, 2011, A Sip of the Chalice, Formal Techniques for Java-like languages

Conference paper

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