A Randomized Algorithm for Two - ProcessWait - Free Test

The work of Herlihy and Plotkin on universal shared objects shows how to implement arbitrary wait-free read-modify-write objects from a bounded number of shared read/write bits. deterministically in a concurrent setting using only atomic shared variables. We present a simple randomized algorithm for 2-process wait-free test-and-set, implemented with two 4-valued single writer single reader atomic variables. The worst-case (over all adversarial schedulers) expected number of steps to execute a test-and-set between two processes is at most 11, while a reset takes exactly 1 step. Based on a nite-state analysis, the proofs of correctness and expected length are compressed into one table.