Resilience of mutual exclusion algorithms to transient memory faults

We study the behavior of mutual exclusion algorithms in the presence of unreliable shared memory subject to transient memory faults. It is well-known that classical 2-process mutual exclusion algorithms, such as Dekker and Peterson's algorithms, are not fault-tolerant; in this paper we ask what degree of fault tolerance can be achieved using the same restricted resources as Dekker and Peterson's algorithms, namely, three binary read/write registers. We show that if one memory fault can occur, it is not possible to guarantee both mutual exclusion and deadlock-freedom using three binary registers; this holds in general when fewer than 2f+1 binary registers are used and f may be faulty. Hence we focus on algorithms that guarantee (a) mutual exclusion and starvation-freedom in fault-free executions, and (b) only mutual exclusion in faulty executions. We show that using only three binary registers it is possible to design an 2-process mutual exclusion algorithm which tolerates a single memory fault in this manner. Further, by replacing one read/write register with a test&set register, we can guarantee mutual exclusion in executions where one variable experiences unboundedly many faults. In the more general setting where up to f registers may be faulty, we show that it is not possible to guarantee mutual exclusion using 2f + 1 binary read/write registers if each faulty register can exhibit unboundedly many faults. On the positive side, we show that an n-variable single-fault tolerant algorithm satisfying certain conditions can be transformed into an ((n-1)f + 1)-variable f-fault tolerant algorithm with the same progress guarantee as the original. In combination with our three-variable algorithm, this implies that there is a (2f+1)-variable mutual exclusion algorithm tolerating a single fault in up to f variables without violating mutual exclusion.