Understanding and Effectively Preventing the ABA Problem in Descriptor-Based Lock-Free Designs

An increasing number of modern real-time systems and the nowadays ubiquitous multicore architectures demand the application of programming techniques for reliable and efficient concurrent synchronization. Some recently developed Compare-And-Swap (CAS) based nonblocking techniques hold the promise of delivering practical and safer concurrency. The ABA problem is a fundamental problem to many CAS-based designs. Its significance has increased with the suggested use of CAS as a core atomic primitive for the implementation of portable lock-free algorithms. The ABA problem's occurrence is due to the intricate and complex interactions of the application's concurrent operations and, if not remedied, ABA can significantly corrupt the semantics of a nonblocking algorithm. The current state of the art leaves the elimination of the ABA hazards to the ingenuity of the software designer. In this work we provide the first systematic and detailed analysis of the ABA problem in lock-free Descriptor-based designs. We study the semantics of Descriptor-based lock-free data structures and propose a classification of their operations that helps us better understand the ABA problem and subsequently derive an effective ABA prevention scheme. Our ABA prevention approach outperforms by a large factor the use of the alternative CAS-based ABA prevention schemes. It offers speeds comparable to the use of the architecture-specific CAS2 instruction used for version counting. We demonstrate our ABA prevention scheme by integrating it into an advanced nonblocking data structure, a lock-free dynamically resizable array.