Practical concurrent traversals in search trees

Abstract

Operations of concurrent objects often employ optimistic concurrency-control schemes that consist of a traversal followed by a validation step. The validation checks if concurrent mutations interfered with the traversal to determine if the operation should proceed or restart. A fundamental challenge is to discover a necessary and sufficient validation check that has to be performed to guarantee correctness. In this paper, we show a necessary and sufficient condition for validating traversals in search trees. The condition relies on a new concept of succinct path snapshots, which are derived from and embedded in the structure of the tree. We leverage the condition to design a general lock-free membership test suitable for any search tree. We then show how to integrate the validation condition in update operations of (non-rebalancing) binary search trees, internal and external, and AVL trees. We experimentally show that our new algorithms outperform existing ones.