Memory models for open-nested transactions

Open nesting provides a loophole in the strict model of atomic transactions. Moss and Hosking suggested adapting open nesting for transactional memory, and Moss and a group at Stanford have proposed hardware schemes to support open nesting. Since these researchers have described their schemes using only operational definitions, however, the semantics of these systems have not been specified in an implementation-independent way. This paper offers a framework for defining and exploring the memory semantics of open nesting in a transactionl-memory setting.Our framework allows us to define the traditional model of serializability and two new transactional-memory models, race freedom and prefix race freedom. The weakest of these memory models, prefix race freedom, closely resembles the Stanford openesting model. We prove that these three memory models are equivalent for transactional-memory systems that support only closed nesting, as long as aborted transactions are "ignored." We prove that for systems that support open nesting, however, the models of serializability, race freedom, and prefix race freedom are distinct. We show that the Stanford TM system implements a model at least as strong as prefix race freedom and strictly weaker than race freedom. Thus, their model compromises serializability, the property traditionally used to reason about the correctness of transactions.