Wait-Free Weak Reference Counting.

Reference counting is a common approach to memory management. One challenge with reference counting is cycles that prevent objects from being deallocated. Systems such as the C++ and Rust standard libraries introduce two types of reference: strong and weak. A strong reference allows access to the object and prevents the object from being deallocated, while a weak reference only prevents deallocation. A weak reference can be upgraded to provide a strong reference provided there are other strong references to the object. Hence, the upgrade operation is partial, and may fail dynamically. The classic implementation of this upgrade operation is not wait-free, that is, it can take arbitrarily long to complete if there is contention on the reference count. In this paper, we propose a wait-free algorithm for weak reference counting. The algorithm requires primitive wait- free atomic operations of “compare and swap”, and “fetch and add”. We provide a correctness proof of the algorithm using the Starling verification tool. We provide a full implementation in C++ and demonstrate the best and worst case performance using micro-benchmarks. For our best case scenario with high contention, our new algorithm provides ~3x more throughput, while for the worst case the new algorithm is ~85% slower. Based on a worst case analysis, we provide a second algorithm that combines the strengths of the two algorithms, and has a significantly improved worst case with ~30% overhead, while maintaining the ~3x speedup for the best case.