Separating Lock-Freedom from Wait-Freedom

Abstract

A long-standing open question has been whether lock-freedom and wait-freedom are fundamentally different progress conditions, namely, can the former be provided in situations where the latter cannot? This paper answers the question in the affirmative, by proving that there are objects with lock-free implementations, but without wait-free implementations-using objects of any finite power. We precisely define an object called n-process long-lived approximate agreement (n-LLAA), in which two sets of processes associated with two sides, 0 or 1, need to decide on a sequence of increasingly closer outputs. We prove that 2-LLAA has a lock-free implementation using reads and writes only, while n-LLAA has a lock-free implementation using reads, writes and (n - 1)-process consensus objects. In contrast, we prove that there is no wait-free implementation of the n-LLAA object using reads, writes and specific (n - 1)-process consensus objects, called (n - 1)-window registers.