An Optimistic Approach to Lock-Free FIFO Queues



FIFO queues are among the most fundamental and highly studied concurrent data structures. The most effective and practical dynamic-memory concurrent queue implementation in the literature is the lock-free FIFO queue algorithm of Michael and Scott, included in the standard Java (TM) Concurrency Package. This paper presents a new dynamic-memory lock-free FIFO queue algorithm that performs consistently better than the Michael and Scott queue. The key idea behind our new algorithm is a novel way of replacing the singly-linked list of Michael and Scott, whose pointers are inserted using a costly compare-and-swap (CAS) operation, by an "optimistic" doubly-linked list whose pointers are updated using a simple store, yet can be "fixed" if a bad ordering of events causes them to be inconsistent. We believe it is the first example of such an "optimistic" approach being applied to a real world data structure.