Thread modularity at many levels: a pearl in compositional verification

A thread-modular proof for the correctness of a concurrent program is based on an inductive and interference-free annotation of each thread. It is well-known that the corresponding proof system is not complete (unless one adds auxiliary variables). We describe a hierarchy of proof systems where each level k corresponds to a generalized notion of thread modularity (level 1 corresponds to the original notion). Each level is strictly more expressive than the previous. Further, each level precisely captures programs that can be proved using uniform Ashcroft invariants with k universal quantifiers. We demonstrate the usefulness of the hierarchy by giving a compositional proof of the Mach shootdown algorithm for TLB consistency. We show a proof at level 2 that shows the algorithm is correct for an arbitrary number of CPUs. However, there is no proof for the algorithm at level 1 which does not involve auxiliary state.