Last-use Opacity: A Strong Safety Property for Transactional Memory with Early Release Support

Transaction Memory (TM) is a concurrency control abstraction that allows the pro-grammer to specify blocks of code to be executed atomically as transactions. However,since transactional code can contain just about any operation attention must be paidto the state of shared variables at any given time. E.g., contrary to a database trans-action, if a TM transaction reads a stale value it may execute dangerous operations,like attempt to divide by zero, access an illegal memory address, or enter an infiniteloop. Thus serializability is insufficient, and stronger safety properties are required inTM, which regulate what values can be read, even by transactions that abort. Hence,a number of TM safety properties were developed, including opacity, and TMS1 andTMS2. However, such strong properties preclude using early release as a techniquefor optimizing TM, because they virtually forbid reading from live transactions. Onthe other hand, properties that do allow early release are either not strong enough toprevent any of the problems mentioned above (recoverability), or add additional con-ditions on transactions with early release that limit their applicability (elastic opacity,live opacity, virtual world consistency). This paper introduces last-use opacity, a newTM safety property that is meant to be a compromise between strong properties likeopacity and serializability. The property eliminates all but a small class of inconsistentviews and poses no stringent conditions on transactions. For illustration, we present alast-use opaque TM algorithm and show that it satisfies the new safety property.