FIFO Read-write Lock uses a counter and a boolean flag to keep track of multiple readers and waiting writer. So, when a writer wants to enter CS, ans new readers are blocked. This prevents writer lock-out due to continual stream of readers. This happens because readers are usually more common.
A common lock is used to ensure internal updates happen atomically and a common condition is used for indicating either "no reader" or "no writer".
Acquiring the read lock involves holding the common lock, waiting until there writer wanting to enter CS (or already entered), and finally incrementing the readers count. Releasing the read lock involves holding the common lock, decrementing the reader count, and signalling any writer/readers.
Acquiring the write lock involves holding the common lock, waiting until there are no writers wanting to enter CS (or already in CS), indicating desire to enter, and finally waiting until there are no readers. Releasing the write lock involves holding the common lock, indicating absence of writer, and signalling any writer/readers.
As mentioned before, this prevents writer lock-out. However, if there are several writers, there is no prioritization among them. This can cause some writers to be waiting for long in the presence of a large number of writers. Thus, this read-write lock is suitable when there are a small number of writers.
Course: Concurrent Data Structures, Monsoon 2020
Taught by: Prof. Govindarajulu Regeti
readLock().lock():
1. Acquire common lock.
2. Wait until no writer wants CS.
3. Increment readers count.
4. Release common lock.readLock().unlock():
1. Acquire common lock.
2. Decrement readers count.
3. If no readers, signal any writer/readers.
4. Release common lock.writeLock().lock():
1. Acquire common lock.
2. Wait until there is no writer.
3. Indicate you want to access CS.
4. Wait until there is no reader.
5. Release common lock.writeLock().unlock():
1. Acquire common lock.
2. Indicate absence of writer.
3. Signal any writer/readers.
4. Release common lock.See FifoReadWriteLock.java for code, Main.java for test, and repl.it for output.