Bounded-Bypass Mutual Exclusion with Minimum Number of Registers

A mutual exclusion mechanism that is both fair and space efficient can be highly valuable for shared memory systems under time and memory constraints such as embedded real-time systems. Several algorithms that utilize only one shared variable and guarantee a certain level of fairness have been proposed. However, these use hypothetical read-modify-write operations that have never been implemented in any system. This paper presents two fair algorithms that do not use such operations, each of which uses a single additional shared variable. The proposed algorithms employ commonly available operations, fetch&store and read/write, on two shared variables. The first algorithm satisfies the bounded-bypass condition. The second is an improvement on the first that satisfies the FIFO condition, which is the most stringent fairness condition. Additionally, it is shown that achieving the bounded-bypass condition using the same set of operations requires two shared variables. Both of the algorithms are thus optimal with respect to the number of shared variables.