Книга: Distributed operating systems

10.2.3. Synchronization

10.2.3. Synchronization

DCE provides two ways for threads to synchronize: mutexes and condition variables. Mutexes are used when it is essential to prevent multiple threads from accessing the same resource at the same time. For example, when moving items around on a linked list, partway through the move, the list will be in an inconsistent state. To prevent disaster, when one thread is manipulating the list, all other threads must be kept away. By requiring a thread to first successfully lock the mutex associated with the list before touching the list (and unlock it afterward), correct operation can be ensured.

Three kinds of mutexes are available, as shown in Fig. 10-5. They differ in the way they deal with nested locks. A fast mutex is analogous to a lock in a data base system. if a process tries to lock an unlocked record, it will succeed. However, if it tries to acquire the same lock a second time, it will block, waiting for the lock to be released, something that will never happen. Deadlock will occur.

Fig. 10-5. Three kinds of mutexes supported by DCE.

A recursive mutex allows a thread to lock a mutex that it has already locked. The idea is this. Suppose that the main program of a thread locks a mutex, then calls a procedure that also locks the mutex. To avoid deadlock, the second lock is accepted. As long as the mutex is ultimately unlocked as many times as it is locked, the nesting can be arbitrarily deep. Although recursive mutexes are more user friendly, they are also considerably slower, so the programmer has to make a choice. As a compromise, DCE provides a third kind of mutex, one in which an attempt to lock a mutex that is already locked does not deadlock, but returns an error instead.

Condition variables provide a second synchronization mechanism. These are used in conjunction with mutexes. Typically, when a thread needs some resource, it uses a mutex to gain exclusive access to a data structure that keeps track of the status of the resource. If the resource is not available, the thread waits on a condition variable, which atomically suspends the thread and releases the mutex. Later, when another thread signals the condition variable, the waiting thread is restarted.

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