When Does a Correct Mutual Exclusion Algorithm Guarantee Mutual Exclusion

Mutual exclusion is usually defined to mean that two processes are not in their critical section at the same time. Something Dan Scales said during a conversation made me suddenly realize that conventional mutual exclusion algorithms do not satisfy that property. I then conjectured how that property could be satisfied, and Perl and Weihl proved that my conjecture was correct. This paper explains why mutual exclusion had not previously been achieved, and how to achieve it–all in less than five pages.