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Temporal logics and model checking for fairly correct systems
 In Proc. 21st Ann. Symp. Logic in Computer Science (LICS’06
, 2006
"... We motivate and study a generic relaxation of correctness of reactive and concurrent systems with respect to a temporal specification. We define a system to be fairly correct if there exists a fairness assumption under which it satisfies its specification. Equivalently, a system is fairly correct if ..."
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We motivate and study a generic relaxation of correctness of reactive and concurrent systems with respect to a temporal specification. We define a system to be fairly correct if there exists a fairness assumption under which it satisfies its specification. Equivalently, a system is fairly correct if the set of runs satisfying the specification is large from a topological point of view, i.e., it is a comeager set. We compare topological largeness with its more popular sibling, probabilistic largeness, where a specification is probabilistically large if the set of runs satisfying the specification has probability 1. We show that topological and probabilistic largeness of ωregular specifications coincide for bounded Borel measures on finitestate systems. As a corollary, we show that, for specifications expressed in LTL or by Büchi automata, checking that a finitestate system is fairly correct has the same complexity as checking that it is correct. Finally we study variants of the logics CTL and CTL*, where the ‘for all runs ’ quantifier is replaced by a ‘for a large set of runs ’ quantifier. We show that the model checking complexity for these variants is the same as for the original logics. 1
Parametric and terminationsensitive control dependence  extended abstract
 In International Static Analysis Symposium (SAS
, 2006
"... Abstract. A parametric approach to control dependence is presented, where the parameter is any prefixinvariant property on paths in the controlflow graph. Existing control dependencies, both direct and indirect, can be obtained as instances of the parametric framework for particular properties on ..."
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Abstract. A parametric approach to control dependence is presented, where the parameter is any prefixinvariant property on paths in the controlflow graph. Existing control dependencies, both direct and indirect, can be obtained as instances of the parametric framework for particular properties on paths. A novel control dependence relation, called terminationsensitive control dependence, is obtained also as an instance of the parametric framework. This control dependence is sensitive to the termination information of loops, which can be given as annotations on loops. If all loops are annotated as terminating then it becomes the classic control dependence, while if all loops are annotated as nonterminating then it becomes the weak control dependence; since in practice some loops are terminating and others are not, terminationsensitive control dependence is expected to improve the precision of analysis tools using it. The unifying formal framework for direct and indirect control dependencies suggests also, in a natural way, a unifying terminology for the various notions of control dependency, which is also proposed in this paper. Finally, a worstcase O(n2) algorithm to compute the indirect terminationsensitive control dependence for languages like Java and C# is given, avoiding the O(n3) complexity of the trivial algorithm calculating the transitive closure of the direct dependence.
Finding State Solutions to Temporal Logic Queries
, 2007
"... Different analysis problems for statetransition models can be uniformly treated as instances of temporal logic querychecking, where only states are sought as solutions to the queries. In this paper, we propose a symbolic querychecking algorithm that finds exactly the state solutions to any query ..."
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Different analysis problems for statetransition models can be uniformly treated as instances of temporal logic querychecking, where only states are sought as solutions to the queries. In this paper, we propose a symbolic querychecking algorithm that finds exactly the state solutions to any query. We show that our approach generalizes previous adhoc techniques, and this generality allows us to find new and interesting applications, such as finding stable states. Our algorithm is linear in the size of the state space and in the cost of model checking, and has been implemented on top of the model checker NuSMV, using the latter as a black box. We show the effectiveness of our approach by comparing it, on a gene network example, to the naive algorithm in which all possible state solutions are checked separately.
TLQ: A Query Solver for States
"... Abstract. In this paper, we present TLQ, a tool that finds the state solutions to any CTL query. It extends existing approaches specialized in finding state solutions to special kinds of queries, being at the same time more efficient than general query checkers that can find all solution to any quer ..."
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Abstract. In this paper, we present TLQ, a tool that finds the state solutions to any CTL query. It extends existing approaches specialized in finding state solutions to special kinds of queries, being at the same time more efficient than general query checkers that can find all solution to any query. Its generality allows its application to new problems, such as finding stable states, as required in some applications from genetics. We describe the implementation of TLQ on top of the modelchecker NuSMV, and show its effectiveness in finding the stable states of a gene network. We describe our tool TLQ that applies to the analysis of statetransition models, and can answer questions of the type: “What are the states that satisfy a property ϕ?”. We address those cases where ϕ cannot be formulated in a temporal logic appropriate for model checking. Instead, ϕ can be formulated as a temporal query suitable for query checking. An example of such a property ϕ is “reachability”. We cannot formulate a CTL property whose modelchecking results in those states reachable from a given state, but we can formulate the CTL query EF? that literally encodes the question:
Constraint based automated synthesis of nonmasking and stabilizing faulttolerance
 in Reliable Distributed Systems, 2009. SRDS ’09. 28th IEEE International Symposium on
, 2009
"... We focus on constraintbased automated addition of nonmasking and stabilizing faulttolerance to hierarchical programs. We specify legitimate states of the program in terms of constraints that should be satisfied in those states. To deal with faults that may violate these constraints, we add recove ..."
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We focus on constraintbased automated addition of nonmasking and stabilizing faulttolerance to hierarchical programs. We specify legitimate states of the program in terms of constraints that should be satisfied in those states. To deal with faults that may violate these constraints, we add recovery actions while ensuring interference freedom among the recovery actions added for satisfying different constraints. Since the constraintbased manual design of faulttolerance is wellknown to be applicable in the manual design of nonmasking faulttolerance, we expect our approach to have a significant benefit in automation of faulttolerant programs. We illustrate our algorithms with three case studies: stabilizing mutual exclusion, stabilizing diffusing computation, and a data dissemination problem in sensor networks. With experimental results, we show that the complexity of synthesis is reasonable and that it can be reduced using the structure of the hierarchical systems. To our knowledge, this is the first instance where automated synthesis has been successfully used in synthesizing programs that are correct under fairness assumptions. Moreover, in two of the case studies considered in this paper, the structure of the recovery paths is too complex to permit existing heuristic based approaches for adding recovery. 1
On Fairness and Randomness
"... We investigate the relation between the behavior of nondeterministic systems under fairness constraints, and the behavior of probabilistic systems. To this end, first a framework based on computable stopping strategies is developed that provides a common foundation for describing both fair and prob ..."
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We investigate the relation between the behavior of nondeterministic systems under fairness constraints, and the behavior of probabilistic systems. To this end, first a framework based on computable stopping strategies is developed that provides a common foundation for describing both fair and probabilistic behavior. On the basis of stopping strategies it is then shown that fair behavior corresponds in a precise sense to random behavior in the sense of MartinLöf’s definition of randomness. nondeterministic systems. Under this perspective the question is investigated what probabilistic properties are needed in such an implementation to guarantee (with probability one) certain required fairness properties in the behavior of the probabilistic system. Generalizing earlier concepts of ɛbounded transition probabilities, we introduce the notion of divergent probabilistic systems, which enables an exact characterization of the fairness properties of a probabilistic implementation. Looking beyond pure fairness properties, we also investigate what other qualitative system properties are guaranteed by probabilistic implementations of fair nondeterministic behavior. This leads to a completeness result which generalizes a wellknown theorem by Pnueli and Zuck.