A Simple Optimistic Skiplist Algorithm

Because of their highly distributed nature and the lack of global re-balancing, skip-lists are becoming an increasingly important logarithmic search structure for concurrent applications. Unfortunately, none of the concurrent skiplist implementations in the literature, whether lock-based or lock-free, have been proven correct. Moreover, the complex structure of these algorithms, most likely the reason for a lack of a proof, is a barrier to software designers that wish to extend and modify the algorithms or base new structures on them. This paper proposes a simple new lock-based concurrent skip-list algorithm. Unlike other concurrent skiplist algorithms, this algorithm preserves the skip-list properties at all times, which facilitates reasoning about its correctness. Though it is lock-based, the algorithm derives a high degree of scalability from a novel use of optimistic synchronization: it searches without acquiring locks, requiring only a short lock-based validation before adding or removing nodes. Experimental evidence shows that this simpler algorithm performs as well as the best previously known lock-free algorithm under the most common search structure usage patterns.