Practical Concurrent Traversals in Search Trees Dana Drachsler-Cohen∗ Martin Vechev Eran Yahav ETH Zurich, Switzerland ETH Zurich, Switzerland Technion, Israel
[email protected] [email protected] [email protected] Abstract efficiently remains a difficult11 task[ ]. This task is partic- Operations of concurrent objects often employ optimistic ularly challenging for search trees whose traversals may concurrency-control schemes that consist of a traversal fol- be performed concurrently with modifications that relocate lowed by a validation step. The validation checks if concur- nodes. Thus, a major challenge in designing a concurrent rent mutations interfered with the traversal to determine traversal operation is to ensure that target nodes are not if the operation should proceed or restart. A fundamental missed, even if they are relocated. This challenge is ampli- challenge is to discover a necessary and sufficient validation fied when a concurrent data structure is optimized forread check that has to be performed to guarantee correctness. operations and traversals are expected to complete without In this paper, we show a necessary and sufficient condi- costly synchronization primitives (e.g., locks, CAS). Recent tion for validating traversals in search trees. The condition work offers various concurrent data structures optimized for relies on a new concept of succinct path snapshots, which are lightweight traversals [1, 2, 4, 6–10, 12–14, 18, 19, 22, 25, 27]. derived from and embedded in the structure of the tree. We However, they are mostly specialized solutions, supporting leverage the condition to design a general lock-free mem- standard operations, that cannot be applied directly to other bership test suitable for any search tree.