General and efficient locking without blocking

Abstract

Standard concurrency control mechanisms offer a trade-off: Transactional memory approaches maximize concurrency, but suffer high overheads and cost for retrying in the case of actual contention. Locking offers lower overheads, but typically reduces concurrency due to the difficulty of associating locks with the exact data that need to be accessed. Moreover, locking allows irreversible operations, is ubiquitous in legacy software, and seems unlikely to ever be completely supplanted. We believe that the trade-off between transactions and (blocking) locks has not been sufficiently exploited to obtain a "best of both worlds" mechanism, although the main components have been identified. Mechanisms for converting locks to atomic sections (which can abort and retry) have already been proposed in the literature: Rajwar and Goodman's "lock elision" (at the hardware level) and Welc et al.'s hybrid monitors (at the software level) are the best known representatives. Nevertheless, these approaches admit improvements on both the generality and the performance front. In this position paper we present two ideas. First, we discuss an adaptive criterion for switching from a locking to a transactional implementation, and back to a locking implementation if the transactional one appears to be introducing overhead for no gain in concurrency. Second, we discuss the issues arising when locks are nested. Contrary to assertions in past work, transforming locks into transactions can be incorrect in the presence of nesting. We explain the problem and provide a precise condition for safety.