Sharedexclusive

Sharedexclusive, typically referred to as a read-write lock or shared-exclusive lock, is a synchronization primitive used to control concurrent access to a shared resource. It allows multiple readers to access the resource simultaneously while ensuring that writers have exclusive access. The fundamental rules are that any number of threads may hold a shared (read) lock, but only one thread may hold an exclusive (write) lock, and when a thread holds the exclusive lock, no other locks may be held.

Common operations include acquiring and releasing shared locks and acquiring and releasing exclusive locks. Many implementations support upgrading a held shared lock to an exclusive lock and downgrading an exclusive lock to a shared one, often with careful ordering to avoid race conditions. The exact API varies by language and library, but the concept remains the same.

Shared-exclusive locks are advantageous when read operations are far more frequent than writes, because they permit high read concurrency without blocking readers unnecessarily. They are widely used in database engines, file systems, and multi-threaded applications to reduce contention.

Popular examples include C++ std::shared_mutex and std::shared_lock, Java's ReentrantReadWriteLock, Python libraries offering reader-writer locks, and Go's sync.RWMutex. Some implementations offer fairness guarantees to avoid writer or reader starvation, while others favor one side, potentially leading to starvation under heavy load.

Disadvantages include added complexity and the risk of deadlocks or priority inversion if not used carefully. Developers must ensure proper lock acquisition order and consider potential contention patterns.