A simple correctness proof of the MCS contention-free lock

Abstract

Mellor-Crummey and Scott present a spin-lock that avoids network contention by having processors spin on local memory locations. Their algorithm is equivalent to a lock-free queue with a special access pattern. The authors provide a complex and unintuitive proof of the correctness of their algorithm. In this paper, we provide a simple proof that the MCS lock is a correct critical section solution, which provides insight into why the algorithm is correct. We show that each acquire-lock and release-lock operation has a decisive instruction which orders the operations with respect to each other.