In computer science, a readers–writer (RW) or shared-exclusive lock (also known as a multiple readers/single-writer lock or multi-reader lock, or push lock) is a synchronization primitive that solves one of the readers–writers problems. An RW lock allows concurrent access for read-only operations, while write operations require exclusive access. This means that multiple threads can read the data in parallel but an exclusive lock is needed for writing or modifying data. When a writer is writing the data, all other writers or readers will be blocked until the writer is finished writing. A common use might be to control access to a data structure in memory that cannot be updated atomically and is invalid (and should not be read by another thread) until the update is complete.
Upgradable RW lockEdit
Some RW locks allow the lock to be atomically upgraded from being locked in read-mode to write-mode, as well as being downgraded from write-mode to read-mode. 
RW locks can be designed with different priority policies for reader vs. writer access. The lock can either be designed to always give priority to readers (read-preferring), to always give priority to writers (write-preferring) or be unspecified with regards to priority. These policies lead to different tradeoffs with regards to concurrency and starvation.
- Read-preferring RW locks allow for maximum concurrency, but can lead to write-starvation if contention is high. This is because writer threads will not be able to acquire the lock as long as at least one reading thread holds it. Since multiple reader threads may hold the lock at once, this means that a writer thread may continue waiting for the lock while new reader threads are able to acquire the lock, even to the point where the writer may still be waiting after all of the readers which were holding the lock when it first attempted to acquire it have released the lock. Priority to readers may be weak, as just described, or strong, meaning that whenever a writer releases the lock, any blocking readers always acquire it next.:76
- Write-preferring RW locks avoid the problem of writer starvation by preventing any new readers from acquiring the lock if there is a writer queued and waiting for the lock; the writer will acquire the lock as soon as all readers which were already holding the lock have completed. The downside is that write-preferring locks allows for less concurrency in the presence of writer threads, compared to read-preferring RW locks. Also the lock is less performant because each operation, taking or releasing the lock for either read or write, is more complex, internally requiring taking and releasing two mutexes instead of one. This variation is sometimes also known as "write-biased" readers–writer lock.
- Unspecified priority RW locks does not provide any guarantees with regards read vs. write access. Unspecified priority can in some situations be preferable if it allows for a more efficient implementation.
Several implementation strategies for readers–writer locks exist, reducing them to synchronization primitives that are assumed to pre-exist.
Using two mutexesEdit
Raynal demonstrates how to implement an R/W lock using two mutexes and a single integer counter. The counter, b, tracks the number of blocking readers. One mutex, r, protects b and is only used by readers; the other, g (for "global") ensures mutual exclusion of writers. This requires that a mutex acquired by one thread can be released by another. The following is pseudocode for the operations:
This implementation strategy gives weak priority to readers.:76
Using a condition variable and a mutexEdit
Alternatively, a read-preferring R/W lock can be implemented in terms of a condition variable and an ordinary (mutex) lock, in addition to an integer counter and a boolean flag. The lock-for-read operation in this setup is:
Each of lock-for-read and lock-for-write has its own inverse operation. Releasing a read lock is done by decrementing r and signalling c if r has become zero (both while holding m), so that one of the threads waiting on c can wake up and lock the R/W lock. Releasing the write lock means setting w to false and broadcasting on c (again while holding m).
Programming language supportEdit
- POSIX standard
pthread_rwlock_tand associated operations
- ReadWriteLock interface and the ReentrantReadWriteLock locks in Java version 5 or above
System.Threading.ReaderWriterLockSlimlock for C# and other .NET languages
std::shared_mutexread/write lock in C++17
boost::upgrade_mutexlocks in Boost C++ Libraries
SRWLock, added to the Windows operating system API as of Windows Vista.
- Phase fair reader–writer lock, which alternates between readers and writers
std::sync::RwLockread/write lock in Rust
- Poco::RWLock in POCO C++ Libraries
txrwlock.ReadersWriterDeferredLockReaders/Writer Lock for Twisted
- This is the standard "wait" operation on condition variables, which, among other actions, releases the mutex m.
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java.util.concurrent.locks.ReentrantReadWriteLockJava readers–writer lock implementation offers a "fair" mode
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