Revisiting Condition Variables and Transactions

Prior condition synchronization primitives for memory transactions either force waiting transactions to abort (the retry construct), or force them to commit (also called punctuation in the literature). Although these primitives are useful in some settings, they do not enable programmers to conveniently express idioms that require synchronous communication (e.g., n-way rendezvous operations) between transactions. We present xCondition, a new form of condition variable that neither forces transactions to abort, nor to commit. Instead, an xCondition creates dependencies between the waiting and the corresponding notifying transactions such that the waiter can commit only if the corresponding notifier commits. If waiters and notifiers form dependency cycles (for instance, in synchronous communication idioms), they must commit or abort together. The xCondition construct builds on our earlier work on transaction communicators. We describe how to use xConditions in conjunction with communicators to enable effective coordination and communication between concurrent transactions. We illustrate the use of xConditions, and describe their implementation in the Maxine VM.