Peterson’s Algorithm in Isabelle/HOL

Peterson’s algorithm [Pet81] is designed to ensure mutual exclusion between two processes without any special hardware support. Peterson’s algorithm has been formalised and verified several times. Although the algorithm is essentially finite state, and therefore susceptible to model checking, we provide an invariant style proof. Invariant style proofs illustrate techniques that are usable for problems that do not fall to model checking techniques. They can also provide important information to justify intuitive notions of why an algorithm is correct, and this knowledge may serve when tackling more complicated algorithms. Finally, invariant style proofs are often more elegant than model checking proofs.