Communicating State Transition Systems for Fine-Grained Concurrent Resources

We present a novel model of concurrent computations with shared memory and provide a simple, yet powerful, logical framework for uniform Hoarestyle reasoning about partial correctness of coarse- and fine-grained concurrent programs. The key idea is to specify arbitrary resource protocols as communicating state transition systems STS that describe valid states of a resource and the transitions the resource is allowed to make, including transfer of heap ownership. We demonstrate how reasoning in terms of communicating STS makes it easy to crystallize behavioral invariants of a resource. We also provide entanglement operators to build large systems from an arbitrary number of STS components, by interconnecting their lines of communication. Furthermore, we show how the classical rules from the Concurrent Separation Logic CSL, such as scoped resource allocation, can be generalized to fine-grained resource management. This allows us to give specifications as powerful as Rely-Guarantee, in a concise, scoped way, and yet regain the compositionality of CSL-style resource management. We proved the soundness of our logic with respect to the denotational semantics of action trees variation on Brookes' action traces. We formalized the logic as a shallow embedding in Coq and implemented a number of examples, including a construction of coarse-grained CSL resources as a modular composition of various logical and semantic components.