A Run-time System for SCOOP

ing locking in terms of fast and slow methods of lock acquisition, where the slow method requires communication between threads on wait queues, is similar to the approach described by Agesen [1]. The list built by the unlocking processor is called the HANDOFF-list and the process of signalling each processor in turn is called the HANDOFF-protocol. Each processor woken during the HANDOFF-protocol has exclusive access to the lock manager despite not currently owning the lock manager mutex. Thus there is always only one thread of execution in the lock manager. This means that actions on the wait queues are implicitly serialized. The order of processors on the wait queues never explicitly changes (processors don’t get a new time stamp while on the wait queues). However, a later processor may overtake an earlier one if it can gain all its resources while the earlier processor is waiting on some other resource. Some resource allocation algorithms do not allow such overtaking, but it is required in this case as the earlier process may be waiting on a lock held by the later one (because of the nested locking), so deadlock would occur if overtaking were not allowed. The pseudo code algorithms for the entry and exit sections are given in Figures 9 and 10. Multiple instructions written on one line, separated by a dash (—), indicate a group of instructions executed atomically. lock_resources(S) /* S is the set of resources to lock */ lock(LockManager) get timestamp i := 0 while i< |S| do i := i + 1 trylock(si) if could not lock si add self to si wait queue for j:= 1 to i do unlock s j end unlock(LockManager) — signal HANDOFF — wait i := 0 end end signal HANDOFF if this subsystem still holds a lock on LockManager unlock(LockManager) end Figure 9: The entry section of the resource allocation algorithm for the SCOOP run-time system. 142 JOURNAL OF OBJECT TECHNOLOGY VOL 1, NO. 3 4 DESIGN AND IMPLEMENTATION unlock_resources(S) /* S is the set of resources to unlock */ lock(LockManager) foreach si ∈ S unlock(si) if no recursive lock is held on si add si wait list to HANDOFF-list end end signal HANDOFF — wait unlock(LockManager) Figure 10: The exit section of the resource allocation algorithm for the SCOOP run-time system.