Reader-writer semaphores, as spin locks of the same nature, should not be used unless there is a clear separation between write paths and read paths in your code. This function atomically converts an acquired write lock to a read lock. Reader-writer semaphores have a unique method that their reader-writer spin lock cousins do not have: downgrade_writer().

Be careful: For admittedly no good reason, this is the opposite of normal semaphore behavior! They both return nonzero if the lock is successfully acquired and zero if it is currently contended. Each has one parameter: a pointer to a reader-writer semaphore. */Īs with semaphores, implementations of down_read_trylock() and down_write_trylock() are provided. * attempt to acquire the semaphore for writing. * attempt to acquire the semaphore for reading.

All reader-writer locks use uninterruptible sleep, so there is only one version of each down(). Conversely, only a sole writer (with no readers) can acquire the write variant of the lock. Any number of readers can concurrently hold the read lock, so long as there are no writers. Reader-writer semaphores that are created dynamically are initialized viaĪll reader-writer semaphores are mutexes (that is, their usage count is one). Where name is the declared name of the new semaphore.

Statically declared reader-writer semaphores are created via Reader-writer semaphores are represented by the struct rw_semaphore type, which is declared in. The situations where reader-writer semaphores are preferred over standard semaphores are the same as with reader-writer spin locks versus standard spin locks. Semaphores, like spin locks, also come in a reader-writer flavor.

Reader writer code