Capability-Based Type Systems for Concurrency Control

Castegren, E. 2018. Capability-Based Type Systems for Concurrency Control. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1611. 106 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0187-7. Since the early 2000s, in order to keep up with the performance predictions of Moore's law, hardware vendors have had to turn to multi-core computers. Today, parallel hardware is everywhere, from massive server halls to the phones in our pockets. However, this parallelism does not come for free. Programs must explicitly be written to allow for concurrent execution, which adds complexity that is not present in sequential programs. In particular, if two concurrent processes share the same memory, care must be taken so that they do not overwrite each other's data. This issue of data-races is exacerbated in object-oriented languages, where shared memory in the form of aliasing is ubiquitous. Unfortunately, most mainstream programming languages were designed with sequential programming in mind, and therefore provide little or no support for handling this complexity. Even though programming abstractions like locks can be used to synchronise accesses to shared memory, the burden of using these abstractions correctly and efficiently is left to the programmer. The contribution of this thesis is programming language technology for controlling concurrency in the presence of shared memory. It is based on the concept of reference capabilities, which facilitate safe concurrent programming by restricting how memory may be accessed and shared. Reference capabilities can be used to enforce correct synchronisation when accessing shared memory, as well as to prevent unsafe sharing when using more finegrained concurrency control, such as lock-free programming. This thesis presents the design of a capability-based type system with low annotation overhead, that can statically guarantee the absence of data-races without giving up object-oriented features like aliasing, subtyping and code reuse. The type system is formally proven safe, and has been implemented for the highly concurrent object-oriented programming language Encore.