Concurrent Reference Counting and Resource Management in Wait-free Constant Time

Abstract: A common problem when implementing concurrent programs is efficiently protecting against unsafe races between processes reading and then using a resource (e.g., memory blocks, file descriptors, or network connections) and other processes that are concurrently overwriting and then destructing the same resource. Such read-destruct races can be protected with locks, or with lock-free solutions such as hazard-pointers or read-copy-update (RCU). In this paper we describe a method for protecting read-destruct races with expected constant time overhead, $O(P^2)$ space and $O(P^2)$ delayed destructs, and with just single word atomic memory operations (reads, writes, and CAS). It is based on an interface with four primitives, an acquire-release pair to protect accesses, and a retire-eject pair to delay the destruct until it is safe. We refer to this as the acquire-retire interface. Using the acquire-retire interface, we develop simple implementations for three common use cases: (1) memory reclamation with applications to stacks and queues, (2) reference counted objects, and (3) objects manage by ownership with moves, copies, and destructs. The first two results significantly improve on previous results, and the third application is original. Importantly, all operations have expected constant time overhead.