ReEnact: using thread-level speculation mechanisms to debug data races in multithreaded codes

While removing software bugs consumes vast amounts of human time, hardware support for debugging in modern computers remains rudimentary. Fortunately, we show that mechanisms for thread level speculation (TLS) can be reused to boost debugging productivity. Most notably, TLS´s rollback capabilities can be extended to support rolling back recent buggy execution and repeating it as many times as necessary until the bug is fully characterized. These incremental reexecutions are deterministic even in multithreaded codes. Importantly, this operation can be done automatically on the fly, and is compatible with production runs. As a specific implementation of a TLS-based debugging framework, we introduce ReEnact. ReEnact targets a particularly hairy class of bugs: data races in multithreaded programs. ReEnact extends the communication monitoring mechanisms in TLS to also detect data races. It extends TLS´s rollback capabilities to be able to roll back and deterministically reexecute the code with races to obtain the race signature. Finally, the signature is compared to a library of race patterns and, if a match occurs, the execution may be repaired. Overall, ReEnact successfully detects, characterizes, and often repairs races automatically on the fly. Moreover, it is fully compatible with always-on use in production runs: the slowdown of race-free execution with ReEnact is on average only 5.8%.