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Invited Talk

**Published in:** LIPIcs, Volume 279, 34th International Conference on Concurrency Theory (CONCUR 2023)

Developing correct and performant concurrent systems is a major challenge. When programming an application using a memory system, a natural expectation would be that each memory update is immediately visible to all concurrent threads (which corresponds to strong consistency). However, for performance reasons, only weaker guarantees can be ensured by memory systems, defined by what sets of updates can be made visible to each thread at any moment, and by the order in which they are made visible. The conditions on the visibility order guaranteed by a memory system corresponds to its memory consistency model. Weak consistency models admit complex and unintuitive behaviors, which makes the task of application programmers extremely hard. It is therefore important to determine an adequate level of consistency for each given application: a level that is weak enough to ensure performance, but also strong enough to ensure correctness of the application behaviors. This leads to the consideration of several important verification problems:
- the correctness of an application program running over a weak consistency model;
- the robustness of an application program w.r.t. consistency weakening;
- the fact that an implementation of a system (memory, storage system) guarantees a given (weak) consistency model.
The talk gives a broad presentation of these issues and some results in this research area. The talk is based on several joint works with students and colleagues during the last few years.

Ahmed Bouajjani. On Verifying Concurrent Programs Under Weakly Consistent Models (Invited Talk). In 34th International Conference on Concurrency Theory (CONCUR 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 279, p. 2:1, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{bouajjani:LIPIcs.CONCUR.2023.2, author = {Bouajjani, Ahmed}, title = {{On Verifying Concurrent Programs Under Weakly Consistent Models}}, booktitle = {34th International Conference on Concurrency Theory (CONCUR 2023)}, pages = {2:1--2:1}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-299-0}, ISSN = {1868-8969}, year = {2023}, volume = {279}, editor = {P\'{e}rez, Guillermo A. and Raskin, Jean-Fran\c{c}ois}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2023.2}, URN = {urn:nbn:de:0030-drops-189961}, doi = {10.4230/LIPIcs.CONCUR.2023.2}, annote = {Keywords: Concurrent programs, weakly consistent models} }

Document

**Published in:** LIPIcs, Volume 140, 30th International Conference on Concurrency Theory (CONCUR 2019)

Distributed storage systems and databases are widely used by various types of applications. Transactional access to these storage systems is an important abstraction allowing application programmers to consider blocks of actions (i.e., transactions) as executing atomically. For performance reasons, the consistency models implemented by modern databases are weaker than the standard serializability model, which corresponds to the atomicity abstraction of transactions executing over a sequentially consistent memory. Causal consistency for instance is one such model that is widely used in practice.
In this paper, we investigate application-specific relationships between several variations of causal consistency and we address the issue of verifying automatically if a given transactional program is robust against causal consistency, i.e., all its behaviors when executed over an arbitrary causally consistent database are serializable. We show that programs without write-write races have the same set of behaviors under all these variations, and we show that checking robustness is polynomial time reducible to a state reachability problem in transactional programs over a sequentially consistent shared memory. A surprising corollary of the latter result is that causal consistency variations which admit incomparable sets of behaviors admit comparable sets of robust programs. This reduction also opens the door to leveraging existing methods and tools for the verification of concurrent programs (assuming sequential consistency) for reasoning about programs running over causally consistent databases. Furthermore, it allows to establish that the problem of checking robustness is decidable when the programs executed at different sites are finite-state.

Sidi Mohamed Beillahi, Ahmed Bouajjani, and Constantin Enea. Robustness Against Transactional Causal Consistency. In 30th International Conference on Concurrency Theory (CONCUR 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 140, pp. 30:1-30:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{beillahi_et_al:LIPIcs.CONCUR.2019.30, author = {Beillahi, Sidi Mohamed and Bouajjani, Ahmed and Enea, Constantin}, title = {{Robustness Against Transactional Causal Consistency}}, booktitle = {30th International Conference on Concurrency Theory (CONCUR 2019)}, pages = {30:1--30:18}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-121-4}, ISSN = {1868-8969}, year = {2019}, volume = {140}, editor = {Fokkink, Wan and van Glabbeek, Rob}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2019.30}, URN = {urn:nbn:de:0030-drops-109321}, doi = {10.4230/LIPIcs.CONCUR.2019.30}, annote = {Keywords: Distributed Databases, Causal Consistency, Model Checking} }

Document

**Published in:** LIPIcs, Volume 118, 29th International Conference on Concurrency Theory (CONCUR 2018)

We address the problem of specifying and verifying quantitative properties of procedural programs. These properties typically involve constraints on the relative cumulated costs of executing various tasks (by invoking for instance some particular procedures) within the scope of the execution of some particular procedure. An example of such properties is "within the execution of each invocation of procedure P, the time spent in executing invocations of procedure Q is less than 20 % of the total execution time". We introduce specification formalisms, both automata-based and logic-based, for expressing such properties, and we study the links between these formalisms and their application in model-checking. On one side, we define Constrained Pushdown Systems (CPDS), an extension of pushdown systems with constraints, expressed in Presburger arithmetics, on the numbers of occurrences of each symbol in the alphabet within invocation intervals (subcomputations between matching pushes and pops), and on the other side, we introduce a higher level specification language that is a quantitative extension of CaRet (the Call-Return temporal logic) called QCaRet where nested quantitative constraints over procedure invocation intervals are expressible using Presburger arithmetics. Then, we investigate (1) the decidability of the reachability and repeated reachability problems for CPDS, and (2) the effective reduction of the model-checking problem of procedural programs (modeled as visibly pushdown systems) against QCaRet formulas to these problems on CPDS.

Mohamed Faouzi Atig, Ahmed Bouajjani, K. Narayan Kumar, and Prakash Saivasan. Verifying Quantitative Temporal Properties of Procedural Programs. In 29th International Conference on Concurrency Theory (CONCUR 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 118, pp. 15:1-15:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{atig_et_al:LIPIcs.CONCUR.2018.15, author = {Atig, Mohamed Faouzi and Bouajjani, Ahmed and Narayan Kumar, K. and Saivasan, Prakash}, title = {{Verifying Quantitative Temporal Properties of Procedural Programs}}, booktitle = {29th International Conference on Concurrency Theory (CONCUR 2018)}, pages = {15:1--15:17}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-087-3}, ISSN = {1868-8969}, year = {2018}, volume = {118}, editor = {Schewe, Sven and Zhang, Lijun}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2018.15}, URN = {urn:nbn:de:0030-drops-95531}, doi = {10.4230/LIPIcs.CONCUR.2018.15}, annote = {Keywords: Verification, Formal Methods, Pushdown systems, Visibly pushdown, Quantitative Temporal Properties} }

Document

**Published in:** LIPIcs, Volume 93, 37th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2017)

In this paper, we consider asynchronous programs consisting of multiple recursive threads running in parallel. Each of the threads is equipped with a multi-set. The threads can create tasks and post them onto the multi-sets or read a task from their own. In addition, they can synchronise through a finite set of locks. In this paper, we show that the reachability problem for such class of asynchronous programs is undecidable even under the nested locking policy. We then show that the reachability problem becomes decidable (Exp-space-complete) when the locks are not allowed to be held across tasks. Finally, we show that the problem is NP-complete when in addition to previous restrictions, threads always read tasks from the same state.

Mohamed Faouzi Atig, Ahmed Bouajjani, K. Narayan Kumar, and Prakash Saivasan. Verification of Asynchronous Programs with Nested Locks. In 37th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 93, pp. 11:1-11:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{atig_et_al:LIPIcs.FSTTCS.2017.11, author = {Atig, Mohamed Faouzi and Bouajjani, Ahmed and Narayan Kumar, K. and Saivasan, Prakash}, title = {{Verification of Asynchronous Programs with Nested Locks}}, booktitle = {37th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2017)}, pages = {11:1--11:14}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-055-2}, ISSN = {1868-8969}, year = {2018}, volume = {93}, editor = {Lokam, Satya and Ramanujam, R.}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2017.11}, URN = {urn:nbn:de:0030-drops-84106}, doi = {10.4230/LIPIcs.FSTTCS.2017.11}, annote = {Keywords: asynchronous programs locks concurrency multi-set pushdown systems, multi-threaded programs, reachability, model checking, verification, nested lockin} }

Document

**Published in:** LIPIcs, Volume 85, 28th International Conference on Concurrency Theory (CONCUR 2017)

Efficient implementations of concurrent objects such as atomic collections are essential to modern computing. Unfortunately their correctness criteria — linearizability with respect to given ADT specifications — are hard to verify. Verifying linearizability is undecidable in general, even on classes of implementations where the usual control-state reachability is decidable. In this work we consider concurrent priority queues which are fundamental to many multi-threaded applications like task scheduling or discrete event simulation, and show that verifying linearizability of such implementations is reducible to control-state reachability. This reduction entails the first decidability results for verifying concurrent priority queues with an unbounded number of threads, and it enables the application of existing safety-verification tools for establishing their correctness.

Ahmed Bouajjani, Constantin Enea, and Chao Wang. Checking Linearizability of Concurrent Priority Queues. In 28th International Conference on Concurrency Theory (CONCUR 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 85, pp. 16:1-16:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{bouajjani_et_al:LIPIcs.CONCUR.2017.16, author = {Bouajjani, Ahmed and Enea, Constantin and Wang, Chao}, title = {{Checking Linearizability of Concurrent Priority Queues}}, booktitle = {28th International Conference on Concurrency Theory (CONCUR 2017)}, pages = {16:1--16:16}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-048-4}, ISSN = {1868-8969}, year = {2017}, volume = {85}, editor = {Meyer, Roland and Nestmann, Uwe}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2017.16}, URN = {urn:nbn:de:0030-drops-78079}, doi = {10.4230/LIPIcs.CONCUR.2017.16}, annote = {Keywords: Concurrency, Linearizability, Model Checking} }

Document

**Published in:** LIPIcs, Volume 59, 27th International Conference on Concurrency Theory (CONCUR 2016)

We address the problem of verifying safety properties of concurrent programs running over the TSO memory model. Known decision procedures for this model are based on complex encodings of store buffers as lossy channels. These procedures assume that the number of processes is fixed. However, it is important in general to prove correctness of a system/algorithm in a parametric way with an arbitrarily large number of processes. In this paper, we introduce an alternative (yet equivalent) semantics to the classical one for the TSO model that is more amenable for efficient algorithmic verification and for extension to parametric verification. For that, we adopt a dual view where load buffers are used instead of store buffers. The flow of information is now from the memory to load buffers. We show that this new semantics allows (1) to simplify drastically the safety analysis under TSO, (2) to obtain a spectacular gain in efficiency and scalability compared to existing procedures, and (3) to extend easily the decision procedure to the parametric case, which allows to obtain a new decidability result, and more importantly, a verification algorithm that is more general and more efficient in practice than the one for bounded instances.

Parosh Aziz Abdulla, Mohamed Faouzi Atig, Ahmed Bouajjani, and Tuan Phong Ngo. The Benefits of Duality in Verifying Concurrent Programs under TSO. In 27th International Conference on Concurrency Theory (CONCUR 2016). Leibniz International Proceedings in Informatics (LIPIcs), Volume 59, pp. 5:1-5:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2016)

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@InProceedings{abdulla_et_al:LIPIcs.CONCUR.2016.5, author = {Abdulla, Parosh Aziz and Atig, Mohamed Faouzi and Bouajjani, Ahmed and Ngo, Tuan Phong}, title = {{The Benefits of Duality in Verifying Concurrent Programs under TSO}}, booktitle = {27th International Conference on Concurrency Theory (CONCUR 2016)}, pages = {5:1--5:15}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-017-0}, ISSN = {1868-8969}, year = {2016}, volume = {59}, editor = {Desharnais, Jos\'{e}e and Jagadeesan, Radha}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2016.5}, URN = {urn:nbn:de:0030-drops-61710}, doi = {10.4230/LIPIcs.CONCUR.2016.5}, annote = {Keywords: Weak Memory Models, Reachability Problem, Parameterized Systems} }

Document

Invited Talk

**Published in:** LIPIcs, Volume 45, 35th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2015)

Efficient implementations of concurrent objects such as semaphores, locks, and atomic collections including stacks and queues are vital to modern computer systems. Programming them is however error prone. To minimize synchronization overhead between concurrent object-method invocations, implementors avoid blocking operations like lock acquisition, allowing methods to execute concurrently. However, concurrency risks unintended inter-operation interference. Their correctness is captured by observational refinement which ensures conformance to atomic reference implementations. Formally, given two libraries L_1 and L_2 implementing the methods of some concurrent object, we say L_1 refines L_2 if and only if every computation of every program using L_1 would also be possible were L_2 used instead.
Linearizability, being an equivalent property, is the predominant proof technique for establishing observational refinement: one shows that each concurrent execution has a linearization which is a valid sequential execution according to a specification, given by an abstract data type or atomic reference implementation.
However, checking linearizability is intrinsically hard. Indeed, even in the case where method implementations are finite-state and object specifications are also finite-state, and when a fixed number of threads (invoking methods in parallel) is considered, the linearizability problem is EXPSPACE-complete, and it becomes undecidable when the number of threads is unbounded. These results show in particular that there is a complexity/decidability gap between the problem of checking linearizability and the problem of
checking reachability (i.e., the dual of checking safety/invariance properties), the latter being, PSPACE-complete and EXPSPACE-complete in the above considered cases, respectively.
We address here the issue of investigating cases where tractable reductions of the observational refinement/linearizability problem to the reachability problem, or dually to invariant checking, are possible. Our aim is (1) to develop algorithmic approaches that avoid a systematic exploration of all possible linearizations of all computations, (2) to exploit existing techniques and tools for
efficient invariant checking to check observational refinement, and (3) to establish decidability and complexity results for significant classes of concurrent objects and data structures.
We present two approaches that we have proposed recently. The first approach introduces a parameterized approximation schema for detecting observational refinement violations. This approach exploits a fundamental property of shared-memory library executions: their histories are interval orders, a special case of partial orders which admit canonical representations in which each operation o is mapped to a positive-integer-bounded interval I(o). Interval orders are equipped with a natural notion of length, which corresponds to the smallest integer constant for which an interval mapping exists. Then, we define a notion of bounded-interval-length analysis, and demonstrate its efficiency, in terms of complexity, coverage, and scalability, for detecting observational refinement bugs.
The second approach focuses on a specific class of abstract data types, including common concurrent objects and data structures such as stacks and queues. We show that for this class of objects, the linearizability problem is actually as hard as the control-state reachability problem. Indeed, we prove that in this case, the
existence of linearizability violations (i.e., finite computations that are not linearizable), can be captured completely by a finite number of finite-state automata, even when an unbounded number of parallel operations is allowed (assuming that libraries are data-independent).

Ahmed Bouajjani, Michael Emmi, Constantin Enea, and Jad Hamza. Checking Correctness of Concurrent Objects: Tractable Reductions to Reachability (Invited Talk). In 35th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 45, pp. 2-4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2015)

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@InProceedings{bouajjani_et_al:LIPIcs.FSTTCS.2015.2, author = {Bouajjani, Ahmed and Emmi, Michael and Enea, Constantin and Hamza, Jad}, title = {{Checking Correctness of Concurrent Objects: Tractable Reductions to Reachability}}, booktitle = {35th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2015)}, pages = {2--4}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-97-2}, ISSN = {1868-8969}, year = {2015}, volume = {45}, editor = {Harsha, Prahladh and Ramalingam, G.}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2015.2}, URN = {urn:nbn:de:0030-drops-56638}, doi = {10.4230/LIPIcs.FSTTCS.2015.2}, annote = {Keywords: Concurrent objects, linearizability, verification, bug detection} }

Document

**Published in:** LIPIcs, Volume 29, 34th International Conference on Foundation of Software Technology and Theoretical Computer Science (FSTTCS 2014)

This paper addresses the reachability problem for pushdown systems communicating via shared memory. It is already known that this problem is undecidable. It turns out that undecidability holds even if the shared memory consists of a single boolean variable. We propose a restriction on the behaviours of such systems, called stage bound, towards decidability. A k stage bounded run can be split into a k stages, such that in each stage there is at most one process writing to the shared memory while any number of processes may read from it. We consider several versions of stage-bounded systems and establish decidability and complexity results.

Mohamed Faouzi Atig, Ahmed Bouajjani, K. Narayan Kumar, and Prakash Saivasan. On Bounded Reachability Analysis of Shared Memory Systems. In 34th International Conference on Foundation of Software Technology and Theoretical Computer Science (FSTTCS 2014). Leibniz International Proceedings in Informatics (LIPIcs), Volume 29, pp. 611-623, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2014)

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@InProceedings{atig_et_al:LIPIcs.FSTTCS.2014.611, author = {Atig, Mohamed Faouzi and Bouajjani, Ahmed and Narayan Kumar, K. and Saivasan, Prakash}, title = {{On Bounded Reachability Analysis of Shared Memory Systems}}, booktitle = {34th International Conference on Foundation of Software Technology and Theoretical Computer Science (FSTTCS 2014)}, pages = {611--623}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-77-4}, ISSN = {1868-8969}, year = {2014}, volume = {29}, editor = {Raman, Venkatesh and Suresh, S. P.}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2014.611}, URN = {urn:nbn:de:0030-drops-48754}, doi = {10.4230/LIPIcs.FSTTCS.2014.611}, annote = {Keywords: Reachability problem, Pushdown systems, Counter systems} }

Document

**Published in:** OASIcs, Volume 13, Annual Doctoral Workshop on Mathematical and Engineering Methods in Computer Science (MEMICS'09) (2009)

We propose a generic framework for reasoning about infinite state systems handling data like integers, booleans etc. and having complex control structures. We consider that configurations of such systems are represented by nested data words, i.e., words of ... words over a potentially infinite data domain. We define a logic called $\ndwl$ allowing to reason about nested data words, and we define rewriting systems called $\ndwrs$ over these nested structures. The rewriting systems are constrained by formulas in the logic specifying the rewriting positions as well as structure/data transformations. We define a fragment $\Sigma_2^*$ of $\ndwl$ with a decidable satisfiability problem. Moreover, we show that the transition relation defined by rewriting systems with $\Sigma_2^*$ constraints can be
effectively defined in the same fragment. These results can be used in the automatization of verification problems such as inductive invariance checking and bounded reachability analysis. Our framework allows to reason about a wide range of concurrent systems including multithreaded programs (with procedure calls, thread creation, global/local variables over infinite data domains, locks, monitors, etc.), dynamic networks of timed systems, cache coherence/mutex/communication protocols, etc.

Ahmed Bouajjani, Cezara Drăgoi, Yan Jurski, and Mihaela Sighireanu. Rewriting Systems over Nested Data Words. In Annual Doctoral Workshop on Mathematical and Engineering Methods in Computer Science (MEMICS'09). Open Access Series in Informatics (OASIcs), Volume 13, pp. 70-79, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2009)

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@InProceedings{bouajjani_et_al:OASIcs:2009:DROPS.MEMICS.2009.2356, author = {Bouajjani, Ahmed and Dr\u{a}goi, Cezara and Jurski, Yan and Sighireanu, Mihaela}, title = {{Rewriting Systems over Nested Data Words}}, booktitle = {Annual Doctoral Workshop on Mathematical and Engineering Methods in Computer Science (MEMICS'09)}, pages = {70--79}, series = {Open Access Series in Informatics (OASIcs)}, ISBN = {978-3-939897-15-6}, ISSN = {2190-6807}, year = {2009}, volume = {13}, editor = {Hlinen\'{y}, Petr and Maty\'{a}\v{s}, V\'{a}clav and Vojnar, Tom\'{a}\v{s}}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/DROPS.MEMICS.2009.2356}, URN = {urn:nbn:de:0030-drops-23567}, doi = {10.4230/DROPS.MEMICS.2009.2356}, annote = {Keywords: Nested data words, rewriting systems, program verification, dynamic and parametrized systems, invariance checking} }

Document

**Published in:** LIPIcs, Volume 2, IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (2008)

Multiset pushdown systems have been introduced by Sen and
Viswanathan as an adequate model for asynchronous programs where
some procedure calls can be stored as tasks to be processed
later. The model is a pushdown system supplied with a multiset of
pending tasks. Tasks may be added to the multiset at each
transition, whereas a task is taken from the multiset only when
the stack is empty. In this paper, we consider an extension of
these models where tasks may be of different priority level, and
can be preempted at any point of their execution by tasks of
higher priority. We investigate the control point reachability
problem for these models. Our main result is that this problem is
decidable by reduction to the reachability problem for a
decidable class of Petri nets with inhibitor arcs. We also
identify two subclasses of these models for which the control
point reachability problem is reducible respectively to the
reachability problem and to the coverability problem for Petri
nets (without inhibitor arcs).

Mohamed Faouzi Atig, Ahmed Bouajjani, and Tayssir Touili. Analyzing Asynchronous Programs with Preemption. In IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science. Leibniz International Proceedings in Informatics (LIPIcs), Volume 2, pp. 37-48, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2008)

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@InProceedings{atig_et_al:LIPIcs.FSTTCS.2008.1739, author = {Atig, Mohamed Faouzi and Bouajjani, Ahmed and Touili, Tayssir}, title = {{Analyzing Asynchronous Programs with Preemption}}, booktitle = {IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science}, pages = {37--48}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-08-8}, ISSN = {1868-8969}, year = {2008}, volume = {2}, editor = {Hariharan, Ramesh and Mukund, Madhavan and Vinay, V}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2008.1739}, URN = {urn:nbn:de:0030-drops-17398}, doi = {10.4230/LIPIcs.FSTTCS.2008.1739}, annote = {Keywords: Multiset Pushdown Systems, Multithreaded Asynchronous Programs, Program verification, Verification algorithms} }

Document

**Published in:** Dagstuhl Seminar Proceedings, Volume 8171, Beyond the Finite: New Challenges in Verification and Semistructured Data (2008)

We propose a new formalism for reasoning about dynamic memory heaps, using monotonic abstraction and symbolic backward reachability analysis. We represent the heaps as graphs, and introduce an ordering on these graphs. This enables us to represent the violation of a given safety property as the reachability of a finitely representable set of bad graphs. We also describe how to symbolically compute the reachable states in the transition system induced by a program.

Parosh Aziz Abdulla, Ahmed Bouajjani, Jonathan Cederberg, Frédéric Haziza, Ran Ji, and Ahmed Rezine. Shape Analysis via Monotonic Abstraction. In Beyond the Finite: New Challenges in Verification and Semistructured Data. Dagstuhl Seminar Proceedings, Volume 8171, pp. 1-11, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2008)

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@InProceedings{azizabdulla_et_al:DagSemProc.08171.3, author = {Aziz Abdulla, Parosh and Bouajjani, Ahmed and Cederberg, Jonathan and Haziza, Fr\'{e}d\'{e}ric and Ji, Ran and Rezine, Ahmed}, title = {{Shape Analysis via Monotonic Abstraction}}, booktitle = {Beyond the Finite: New Challenges in Verification and Semistructured Data}, pages = {1--11}, series = {Dagstuhl Seminar Proceedings (DagSemProc)}, ISSN = {1862-4405}, year = {2008}, volume = {8171}, editor = {Anca Muscholl and Ramaswamy Ramanujam and Micha\"{e}l Rusinowitch and Thomas Schwentick and Victor Vianu}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/DagSemProc.08171.3}, URN = {urn:nbn:de:0030-drops-15590}, doi = {10.4230/DagSemProc.08171.3}, annote = {Keywords: Shape analysis, Program verification, Static analysis} }

Document

**Published in:** Dagstuhl Seminar Proceedings, Volume 6081, Software Verification: Infinite-State Model Checking and Static Program Analysis (2006)

From 19.02.06 to 24.02.06, the Dagstuhl Seminar 06081 ``Software Verification: Infinite-State Model Checking and Static Program Analysis'' was held in the International Conference and Research Center (IBFI), Schloss Dagstuhl.
During the seminar, several participants presented their current
research, and ongoing work and open problems were discussed. Abstracts of
the presentations given during the seminar
are put together in this paper. The first section
describes the seminar topics and goals in general.
Links to extended abstracts or full papers are provided, if available.

Parosh Aziz Abdulla, Ahmed Bouajjani, and Markus Müller-Olm. 06081 Abstracts Collection – Software Verification: Infinite-State Model Checking and Static Program Analysis. In Software Verification: Infinite-State Model Checking and Static Program Analysis. Dagstuhl Seminar Proceedings, Volume 6081, pp. 1-18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2006)

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@InProceedings{abdulla_et_al:DagSemProc.06081.1, author = {Abdulla, Parosh Aziz and Bouajjani, Ahmed and M\"{u}ller-Olm, Markus}, title = {{06081 Abstracts Collection – Software Verification: Infinite-State Model Checking and Static Program Analysis}}, booktitle = {Software Verification: Infinite-State Model Checking and Static Program Analysis}, pages = {1--18}, series = {Dagstuhl Seminar Proceedings (DagSemProc)}, ISSN = {1862-4405}, year = {2006}, volume = {6081}, editor = {Parosh Aziz Abdulla and Ahmed Bouajjani and Markus M\"{u}ller-Olm}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/DagSemProc.06081.1}, URN = {urn:nbn:de:0030-drops-7997}, doi = {10.4230/DagSemProc.06081.1}, annote = {Keywords: Software verification, infinite-state systems, static program analysis, automatic analysis} }

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**Published in:** Dagstuhl Seminar Proceedings, Volume 6081, Software Verification: Infinite-State Model Checking and Static Program Analysis (2006)

{\em Software systems} are present at the very heart of many daily-life
applications,
such as in communication (telephony and mobile Internet),
transportation, energy, health, etc.
Such systems are very often {\em critical}\/ in the sense that their failure
can have considerable human/economical consequences.
In order to ensure reliability,
development methods must include {\em algorithmic analysis and verification
techniques}
which allow (1) to detect automatically defective behaviors of the system
and to analyze their source, and (2) to check that every component of a
system
conforms to its specification.
Two important and quite active research communities are particularly
concerned with this challenge:
the community of computer-aided verification, especially the community of
(infinite-state) model checking, and the community of static program
analysis.
From 19.02.06 to 24.02.06,
51 researchers from these two communities
met at the
Dagstuhl Seminar 06081 ``Software Verification: Infinite-State Model Checking and Static Program Analysis''
in order to improve and deepen the mutual
understanding of the developed technologies and to trigger
collaborations.
During the seminar which was held at the
International Conference and Research Center (IBFI),
Schloss Dagstuhl,
several participants presented their current research, and ongoing
work and open problems were discussed. Abstracts of the presentations
given during the seminar are put together in this paper.

Parosh Aziz Abdulla, Ahmed Bouajjani, and Markus Müller-Olm. 06081 Executive Summary – Software Verification: Infinite-State Model Checking and Static Program Analysis. In Software Verification: Infinite-State Model Checking and Static Program Analysis. Dagstuhl Seminar Proceedings, Volume 6081, pp. 1-5, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2006)

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@InProceedings{abdulla_et_al:DagSemProc.06081.2, author = {Abdulla, Parosh Aziz and Bouajjani, Ahmed and M\"{u}ller-Olm, Markus}, title = {{06081 Executive Summary – Software Verification: Infinite-State Model Checking and Static Program Analysis}}, booktitle = {Software Verification: Infinite-State Model Checking and Static Program Analysis}, pages = {1--5}, series = {Dagstuhl Seminar Proceedings (DagSemProc)}, ISSN = {1862-4405}, year = {2006}, volume = {6081}, editor = {Parosh Aziz Abdulla and Ahmed Bouajjani and Markus M\"{u}ller-Olm}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/DagSemProc.06081.2}, URN = {urn:nbn:de:0030-drops-7973}, doi = {10.4230/DagSemProc.06081.2}, annote = {Keywords: Infinite-state systems, model checking, program analysis, software verification} }

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**Published in:** Dagstuhl Seminar Proceedings, Volume 6081, Software Verification: Infinite-State Model Checking and Static Program Analysis (2006)

We introduce asynchronous dynamic pushdown networks (ADPN), a new model for multithreaded programs in which pushdown systems communicate via shared memory. ADPN generalizes both CPS (concurrent pushdown systems) and DPN (dynamic pushdown networks). We show that ADPN exhibit several advantages as a program model. Since the reachability problem for ADPN is undecidable even in the case without dynamic creation of processes, we address the bounded reachability problem, which considers only those computation sequences where the (index of the)
thread accessing the shared memory is changed at most a fixed given
number of times. We provide efficient algorithms for both forward and
backward reachability analysis. The algorithms are based on automata techniques for symbolic representation of sets of configurations.
This talk is based on joint work with Ahmed Bouajjani, Javier Esparza, and Jan Strejcek that appeared in FSTTCS 2005.

Ahmed Bouajjani, Javier Esparza, Stefan Schwoon, and Jan Strejcek. Reachability analysis of multithreaded software with asynchronous communication. In Software Verification: Infinite-State Model Checking and Static Program Analysis. Dagstuhl Seminar Proceedings, Volume 6081, pp. 1-18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2006)

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@InProceedings{bouajjani_et_al:DagSemProc.06081.6, author = {Bouajjani, Ahmed and Esparza, Javier and Schwoon, Stefan and Strejcek, Jan}, title = {{Reachability analysis of multithreaded software with asynchronous communication}}, booktitle = {Software Verification: Infinite-State Model Checking and Static Program Analysis}, pages = {1--18}, series = {Dagstuhl Seminar Proceedings (DagSemProc)}, ISSN = {1862-4405}, year = {2006}, volume = {6081}, editor = {Parosh Aziz Abdulla and Ahmed Bouajjani and Markus M\"{u}ller-Olm}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/DagSemProc.06081.6}, URN = {urn:nbn:de:0030-drops-7263}, doi = {10.4230/DagSemProc.06081.6}, annote = {Keywords: Model checking, pushdown systems, concurrency} }

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**Published in:** Dagstuhl Seminar Reports. Dagstuhl Seminar Reports, Volume 1 (2021)

Ahmed Bouajjani and Javier Esparza. Verification of Infinite-state Systems (Dagstuhl Seminar 00141). Dagstuhl Seminar Report 271, pp. 1-24, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2000)

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@TechReport{bouajjani_et_al:DagSemRep.271, author = {Bouajjani, Ahmed and Esparza, Javier}, title = {{Verification of Infinite-state Systems (Dagstuhl Seminar 00141)}}, pages = {1--24}, ISSN = {1619-0203}, year = {2000}, type = {Dagstuhl Seminar Report}, number = {271}, institution = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/DagSemRep.271}, URN = {urn:nbn:de:0030-drops-151568}, doi = {10.4230/DagSemRep.271}, }

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