Stateless model checking of the Linux kernel\u2019s read\u2013copy update (RCU)

Abstract

Read–copy update (RCU) is a synchronization mechanism used heavily in key components of the Linux kernel, such as the virtual filesystem (VFS), to achieve scalability by exploiting RCU’s ability to allow concurrent reads and updates. RCU’s design is non-trivial, requires a significant effort to fully understand it, let alone become convinced that its implementation is faithful to its specification and provides its claimed properties. The fact that as time goes by Linux kernels are becoming increasingly more complex and are employed in machines with more and more cores and weak memory does not make the situation any easier. This article presents an approach to systematically test the code of the main implementation of RCU used in the Linux kernel (Tree RCU) for concurrency errors, both under sequentially consistent and weak memory. Our modeling allows Nidhugg, a stateless model checking tool, to reproduce, within seconds, safety and liveness bugs that have been reported for RCU. Additionally, we present the real cause behind some failures that have been observed in production systems in the past. More importantly, we were able to verify both the publish–subscribe and the grace-period guarantee, with the latter being the basic and most important guarantee that RCU offers, on several Linux kernel versions, for particular configurations. Our approach is effective, both in dealing with the increased complexity of recent Linux kernels and in terms of time that the process requires. We hold that our effort constitutes a good first step toward making tools such as Nidhugg part of the standard testing infrastructure of the Linux kernel.