Python
A reader-writer lock for Python
Python offers a number of useful synchronization primitives in the threading and Queue modules. One that is missing, however, is a simple reader-writer lock (RWLock). A RWLock allows improved concurrency over a simple mutex, and is useful for objects that have high read-to-write ratios like database caches.
Surprisingly, I haven't been able to find any implementation of these semantics, so I rolled my own in a module rwlock.py to implement a RWLock class, along with lock promotion/demotion. Hopefully it can be added to the standard library threading module. This code is hereby placed in the public domain.
"""Simple reader-writer locks in Python
Many readers can hold the lock XOR one and only one writer"""
import threading
version = """$Id: 04-1.html,v 1.3 2006/12/05 17:45:12 majid Exp $"""
class RWLock:
"""
A simple reader-writer lock Several readers can hold the lock
simultaneously, XOR one writer. Write locks have priority over reads to
prevent write starvation.
"""
def __init__(self):
self.rwlock = 0
self.writers_waiting = 0
self.monitor = threading.Lock()
self.readers_ok = threading.Condition(self.monitor)
self.writers_ok = threading.Condition(self.monitor)
def acquire_read(self):
"""Acquire a read lock. Several threads can hold this typeof lock.
It is exclusive with write locks."""
self.monitor.acquire()
while self.rwlock < 0 or self.writers_waiting:
self.readers_ok.wait()
self.rwlock += 1
self.monitor.release()
def acquire_write(self):
"""Acquire a write lock. Only one thread can hold this lock, and
only when no read locks are also held."""
self.monitor.acquire()
while self.rwlock != 0:
self.writers_waiting += 1
self.writers_ok.wait()
self.writers_waiting -= 1
self.rwlock = -1
self.monitor.release()
def promote(self):
"""Promote an already-acquired read lock to a write lock
WARNING: it is very easy to deadlock with this method"""
self.monitor.acquire()
self.rwlock -= 1
while self.rwlock != 0:
self.writers_waiting += 1
self.writers_ok.wait()
self.writers_waiting -= 1
self.rwlock = -1
self.monitor.release()
def demote(self):
"""Demote an already-acquired write lock to a read lock"""
self.monitor.acquire()
self.rwlock = 1
self.readers_ok.notifyAll()
self.monitor.release()
def release(self):
"""Release a lock, whether read or write."""
self.monitor.acquire()
if self.rwlock < 0:
self.rwlock = 0
else:
self.rwlock -= 1
wake_writers = self.writers_waiting and self.rwlock == 0
wake_readers = self.writers_waiting == 0
self.monitor.release()
if wake_writers:
self.writers_ok.acquire()
self.writers_ok.notify()
self.writers_ok.release()
elif wake_readers:
self.readers_ok.acquire()
self.readers_ok.notifyAll()
self.readers_ok.release()
if __name__ == '__main__':
import time
rwl = RWLock()
class Reader(threading.Thread):
def run(self):
print self, 'start'
rwl.acquire_read()
print self, 'acquired'
time.sleep(5)
print self, 'stop'
rwl.release()
class Writer(threading.Thread):
def run(self):
print self, 'start'
rwl.acquire_write()
print self, 'acquired'
time.sleep(10)
print self, 'stop'
rwl.release()
class ReaderWriter(threading.Thread):
def run(self):
print self, 'start'
rwl.acquire_read()
print self, 'acquired'
time.sleep(5)
rwl.promote()
print self, 'promoted'
time.sleep(5)
print self, 'stop'
rwl.release()
class WriterReader(threading.Thread):
def run(self):
print self, 'start'
rwl.acquire_write()
print self, 'acquired'
time.sleep(10)
print self, 'demoted'
rwl.demote()
time.sleep(10)
print self, 'stop'
rwl.release()
Reader().start()
time.sleep(1)
Reader().start()
time.sleep(1)
ReaderWriter().start()
time.sleep(1)
WriterReader().start()
time.sleep(1)
Reader().start()
