The mutual exclusion problem for unreliable processes

Consider n processes operating asynchronously in parallel, each of wich maintains a single "special" variable which can be read (but not written) by the other processes. All coordination between processes is to be accomplished by means of the execution of the primitive operations of a process (1) reading another process's special variable, and (2) setting its own special variable to some value. A process may "die" at any time, when its special variable is (automatically) set a special "dead" value. A dead process may revive. Reading a special variable which is being simultaneously written returns either the old or the new value. Each process may be in a certain "critical" state (which it leaves if it dies). We present a coordination scheme with the following properties. (1) At most one process is ever in its critical state at a time. (2) If a process wants to enter its critical state, it may do so before any other process enters its critical state more than once. (3) The special variables are bounded in value. (4) Some process wanting to enter its critical state can always make progress to that goal. By the definition of the problem, no process can prevent another from entering its critical state by repeatedly failing and restarting. In the case of two processes, what makes our solution of particular interest is its remarkable simplicity when compared with the extant solutions to this problem. Our n-process solution uses the two-process solution as a subroutine, and is not quite as elegant as the two-process solution.