Readwritelås

Readwritelås, or read-write locks, are a synchronization primitive used to protect shared data while allowing greater parallelism for reads. They distinguish between a shared read access and an exclusive write access: multiple threads may hold the read lock simultaneously, but only one thread may hold the write lock, and the write lock can be held only when no readers are active.

In operation, a thread that acquires the read lock can proceed as long as no thread holds the write lock. If a writer is waiting, some implementations keep accepting new readers (reader-friendly), while others prioritize the writer to avoid starvation (writer-friendly). Writers obtain the lock only when there are no active readers, and once a writer holds the lock, no other reader or writer can access the resource until it is released.

Variations exist to address fairness, performance, and complexity. Some read-write locks support upgrade and downgrade paths (for example, upgrading a read lock to a write lock or downgrading a write lock to a read lock). Non-blocking try-lock variants allow threads to attempt acquisition without waiting. Optimistic read techniques may offer light-weight reads with subsequent validation.

Common usage scenarios include read-heavy data structures, caches, indices, and configuration data where concurrent reads are frequent but writes are infrequent or isolated. Implementations appear in many languages, such as Java’s ReentrantReadWriteLock, C++’s std::shared_mutex, and various libraries for Python and other ecosystems.

Key considerations when using readwritelås include potential writer starvation, deadlock risks if locks are misused, and the overhead of maintaining lock state. Proper design choices regarding fairness and access patterns are essential to reap the benefits of this synchronization primitive.