A mutex is basically a lock that we set (lock) before accessing a shared resource and release (unlock) when we’re done. While it is set, any other thread that tries to set it will block until we release it. If more than one thread is blocked when we unlock the mutex, then all threads blocked on the lock will be made runnable, and the first one to run will be able to set the lock. The others will see that the mutex is still locked and go back to waiting for it to become available again. In this way, only one thread will proceed at a time.
This mutual-exclusion mechanism works only if we design our threads to follow the same data-access rules. The operating system doesn’t serialize access to data for us. If we allow one thread to access a shared resource without first acquiring a lock, then inconsistencies can occur even though the rest of our threads do acquire the lock before attempting to access the shared resource.
Reader–writer locks are similar to mutexes, except that they allow for higher degrees of parallelism. With a mutex, the state is either locked or unlocked, and only one thread can lock it at a time. Three states are possible with a reader–writer lock: locked in read mode, locked in write mode, and unlocked. Only one thread at a time can hold a reader–writer lock in write mode, but multiple threads can hold a reader–writer lock in read mode at the same time.
When a reader–writer lock is write locked, all threads attempting to lock it block until it is unlocked. When a reader–writer lock is read locked, all threads attempting to lock it in read mode are given access, but any threads attempting to lock it in write mode block until all the threads have released their read locks. Although implementations vary, reader–writer locks usually block additional readers if a lock is already held in read mode and a thread is blocked trying to acquire the lock in write mode. This prevents a constant stream of readers from starving waiting writers.
Reader–writer locks are well suited for situations in which data structures are read more often than they are modified. When a reader–writer lock is held in write mode, the data structure it protects can be modified safely, since only one thread at a time can hold the lock in write mode. When the reader–writer lock is held in read mode, the data structure it protects can be read by multiple threads, as long as the threads first acquire the lock in read mode.
Reader–writer locks are also called shared–exclusive locks. When a reader–writer lock is read locked, it is said to be locked in shared mode. When it is write locked, it is said to be locked in exclusive mode
A spin lock is like a mutex, except that instead of blocking a process by sleeping, the process is blocked by busy-waiting (spinning) until the lock can be acquired. A spin lock could be used in situations where locks are held for short periods of times and threads don’t want to incur the cost of being descheduled.
Spin locks are often used as low-level primitives to implement other types of locks. Depending on the system architecture, they can be implemented efficiently using test- and-set instructions. Although efficient, they can lead to wasting CPU resources: while a thread is spinning and waiting for a lock to become available, the CPU can’t do anything else. This is why spin locks should be held only for short periods of time.
Barriers are a synchronization mechanism that can be used to coordinate multiple threads working in parallel. A barrier allows each thread to wait until all cooperating threads have reached the same point, and then continue executing from there. We’ve already seen one form of barrier—the pthread_join function acts as a barrier to allow one thread to wait until another thread exits. Barrier objects are more general than this, however. They allow an arbitrary number of threads to wait until all of the threads have completed processing, but the threads don’t have to exit. They can continue working after all threads have reached the barrier.
- Advanced Programming in the UNIX Environment