Pathological Interaction of Locks with Transactional Memory

Transactional memory (TM) promises to simplify multithreaded programming. Transactions provide mutual exclusion without the possibility of deadlock and the need to assign locks to data structures. To date, most investigations of transactional memory have looked at purely transactional systems that do not interact with legacy code using locks. Unfortunately, the reality of software engineering is that such interaction is likely. We investigate the interaction of transactional memory implementations and lock-based code. We identify and discuss five pathologies that arise with different systems when a lock is accessed both within and outside a transaction: Blocking, Deadlock, Livelock, Early Release, and Invisible Locking. To address these pathologies we designed and implemented transaction-safe locks (TxLocks) by modifying the existing lock implementation of the OpenSolaris C Library and extending the conflict resolution policy of a hardware transactional memory system.