Abstract: In a data processing system a number of processing nodes share resources. Access to the shared resources is controlled by semaphores, each node having a local copy of all the semaphores. Nodes may acquire ownership of semaphores. When a node requires a semaphore operation on a particular semaphore, a semaphore message is broadcast to all the nodes instructing them to perform the semaphore operation on their local copies of the semaphore. If the semaphore is unowned, the node must suspend the semaphore operation until the message returns, so as to ensure correct chronology for the semaphore operation. If, however, the semaphore owned by this node, the node can perform the semaphore operation without waiting for the message to return. This speeds up the semaphore mechanism. If the semaphore is owned by another node, that other node relinquishes ownership so that the semaphore operation can be performed.