Context-aware counter abstraction

The trend towards multi-core computing has made concurrent software an important target of computer-aided verification. Unfortunately, Model Checkers for such software suffer tremendously from combinatorial state space explosion. We show how to apply counter abstraction to real-world concurrent programs to factor out redundancy due to thread replication. The traditional global state representation as a vector of local states is replaced by a vector of thread counters, one per local state. In practice, straightforward implementations of this idea are unfavorably sensitive to the number of local states. We present a novel symbolic exploration algorithm that avoids this problem by carefully scheduling which counters to track at any moment during the search. We have carried out experiments on Boolean programs, an abstraction promoted by the success of the Slam project. The experiments give evidence of the applicability of our method to realistic programs, and of the often huge savings obtained in comparison to plain symbolic state space exploration, and to exploration optimized by partial-order methods. To our knowledge, our tool marks the first implementation of counter abstraction to programs with non-trivial local state spaces, resulting in a Model Checker for concurrent Boolean programs that promises true scalabilit

Sequentializing Parameterized Programs

We exhibit assertion-preserving (reachability preserving) transformations from parameterized concurrent shared-memory programs, under a k-round scheduling of processes, to sequential programs. The salient feature of the sequential program is that it tracks the local variables of only one thread at any point, and uses only O(k) copies of shared variables (it does not use extra counters, not even one counter to keep track of the number of threads). Sequentialization is achieved using the concept of a linear interface that captures the effect an unbounded block of processes have on the shared state in a k-round schedule. Our transformation utilizes linear interfaces to sequentialize the program, and to ensure the sequential program explores only reachable states and preserves local invariants.Comment: In Proceedings FIT 2012, arXiv:1207.348

SAT-based Summarization for Boolean Programs

Abstract. Boolean programs are frequently used to model abstractions of software programs. They have the advantage that reachability properties are decidable, despite the fact that their stack is not bounded. The enabling technique is summarization of procedure calls. Most model checking tools for Boolean programs use BDDs to represent these summaries, allowing for an efficient fix-point detection. However, BDDs are highly sensitive to the number of state variables. We present an approach to over-approximate summaries using Bounded Model Checking. Our technique is based on a SAT solver and requires only few calls to a QBF solver for fix-point detection. We present benchmarks that show that our implementation is able handle a larger number of variables than BDD-based algorithms on some examples.

Biogenesis and structure of a Type VI secretion membrane core complex

International audienceBacteria share their ecological niches with other microbes. The bacterial Type VI secretion system is one of the key players for microbial competition, as well as an important virulence determinant during bacterial infections. It assembles a nano-crossbow-like structure that propels an arrow made of Hcp tube and VgrG spike into the cytoplasm of the attacker cell and punctures the prey's cell wall. The nano-crossbow is stably anchored to the cell envelope of the attacker by a membrane core complex. Here, we show that this complex is assembled by the sequential addition of three proteins-TssJ, TssM and TssL-and present a 11.6 Å resolution structure of the fully assembled complex, determined by negative stain electron microscopy. With overall C5 symmetry, this 1.7-megadalton complex comprises a large base in the cytoplasm. It extends in the periplasm via 10 arches to form a double-ring structure containing the C-terminal domain of TssM (TssM ct) and TssJ that is anchored in the outer membrane. The crystal structure of the TssM ct-TssJ complex coupled to whole-cell accessibility studies suggest that large conformational changes induce transient pore formation in the outer membrane allowing passage of the attacking Hcp tube/VgrG spike