Flexible access control policy for SCOOP

Abstract

The SCOOP model extends Eiffel to support the construction of concurrent applications with little more effort than sequential ones. The model provides strong safety guarantees: mutual exclusion and atomicity at the routine level, and FIFO scheduling of clients’ calls. Unfortunately, in the original proposal of the model (SCOOP_97) these guarantees come at a high price: they entail locking all the arguments of a feature call, even if the corresponding objects are never used by the feature. In most cases, the amount of locking is higher than necessary. Additionally, a client that holds a lock on a given processor cannot relinquish it temporarily when the lock is needed by one of its suppliers. This increases the likelihood of deadlocks; additionally, some interesting synchronisation scenarios, e.g. separate callbacks, cannot be implemented. We propose two refinements of the access control policy for SCOOP: a type-based mechanism to specify which arguments of a routine call should be locked, and a lock passing mechanism for safe handling of complex synchronisation scenarios with mutual locking of several separate objects. When combined, these refinements increase the expressive power of the model, give programmers more control over the computation, and enable more potential parallelism, thus reducing the risk of deadlock.