Checking linearizability of encapsulated extended operations

Linearizable objects (data-structures) provide operations that appear to execute atomically. Modern mainstream languages provide many linearizable data-structures, simplifying concurrent programming. In practice, however, programmers often find a need to execute a sequence of operations (on linearizable objects) that executes atomically and write extended operations for this purpose. Such extended operations are a common source of atomicity bugs. This paper focuses on the problem of verifying that a set of extension operations (to a linearizable library) are themselves linearizable. We present several reduction theorems that simplify this verification problem enabling more efficient verification. We first introduce the notion of an encapsulated extension: this is an extension that (a) does not introduce new shared state (beyond the shared state in the base linearizable library), and (b) accesses or modifies the shared state only through the base operations. We show that encapsulated extensions are widely prevalent in real applications. We show that linearizability of encapsulated extended operations can be verified by considering only histories with one occurrence of an extended operation, interleaved with atomic occurrences of base and extended operations. As a consequence, this verification needs to consider only histories with two threads, whereas general linearizability verification requires considering histories with an unbounded number of threads. We show that when the operations satisfy certain properties, each extended operation can be verified independently of the others, enabling further reductions. We have implemented a simple static analysis algorithm that conservatively verifies linearizabilty of encapsulated extensions of Java concurrent maps. We present empirical results illustrating the benefits of the reduction theorems.