Nothing Is Shared Until You Synchronise: Reinterpreting Java for Data-Race Freedom

Luke Cheeseman, Elias Castegren, David Black-Schaffer, Erik Österlund, Tobias Wrigstad

In mainstream shared-memory languages, programmers use synchronisation mechanisms such as locks to isolate shared data accesses across threads to avoid data races. Yet synchronisation enforces only a temporal ordering, while shared memory still communicates updates between threads at every ordinary read and write. To achieve isolation, programmers therefore need to understand the connection between ordering and communication, and to apply synchronisation correctly and consistently, while compilers and hardware optimise assuming they have. When programmers synchronise incorrectly, they lose isolation, and the semantics of their programs change without notice. We propose Sync-as-Share, a memory model built on isolation and communication rather than on the ordering of accesses to shared memory. We start from thread isolation, where each thread reads and writes a private view of memory, and reinterpret programs' existing synchronisation actions to also communicate updates explicitly. As nothing is communicated outside synchronisation actions, data races become impossible by construction. Instead, conflicting updates are detected at synchronisation points. Viewing synchronisation as…