Transactional memory (TM) eliminates many problems associated with lock-based synchronization. Over recent years, much progress has been made in software and hardware implementation techniques for TM. However, before transactional memory can be integrated into mainstream programming languages, we must precisely define its meaning in the context of these languages. In particular, TM semantics should address the advanced features present in the existing software TM implementations, such as interactions between transactions and locks, explicit user-level abort and Support for legacy code. In this paper, we address these topics from both theoretical and practical points of view. We give precise formulations of several popular TM semantics for the domain of sequentially consistent executions and show that some of these semantics are equivalent for C++ programs that do not contain other forms of synchronization. We show that lock-based semantics, such as Single Global Lock Atomicity (SLA) or Disjoint Lock Atomicity (DLA), do not actually guarantee atomicity for race-free programs and propose a new semantics, Race-Free Atomicity (RFA) that gives such a guarantee. We compare these semantics from the programmer and implementation points of view and explain why supporting non-atomic transactions is useful. Finally, we propose a new set of language constructs that let programmers explicitly specify whether transactions should be atomic and describe how these constructs interact with user-level abort and legacy code.
