Readers–writer lock

In computer science, a readers–writer (single-writer lock, a multi-reader lock, a push lock, or an MRSW lock) is a synchronization primitive that solves one of the readers–writers problems. An RW lock allows concurrent access for read-only operations, whereas 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 and 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.

Readers–writer locks are usually constructed on top of mutexes and condition variables, or on top of semaphores.

Upgradable RW lock

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. Upgrading a lock from read-mode to write-mode is prone to deadlocks, since whenever two threads holding reader locks both attempt to upgrade to writer locks, a deadlock is created that can only be broken by one of the threads releasing its reader lock. The deadlock can be avoided by allowing only one thread to acquire the lock in "read-mode with intent to upgrade to write" while there are no threads in write mode and possibly non-zero threads in read-mode.

Priority policies

RW locks can be designed with different priority policies for reader vs.