我使用Python的并发原语编写了一个读写锁(我想!)。我在SO或其他地方读过的每个实现似乎都使用了2个锁 - 一个用于读取,另一个用于写入。我的实现只包含一个读取监视器,但我可能会遗漏一些关键的东西 - 任何人都可以确认这将起作用吗?如果是这样,使用额外的写锁定有什么好处?
这是经典的读写锁,优先考虑读者(可能会使作家们挨饿)。我使用虚拟缓存来演示读写。
import threading as t
class ReadWriteCache(object):
def __init__(self):
self.cache = {}
self.reads = 0
self.read_cond = t.Condition(t.Lock())
def read(self, key):
with self.read_cond: # Register the read, so writes will wait()
self.reads += 1
result = self.cache[key]
with self.read_cond:
self.reads -= 1
if not self.reads:
self.read_cond.notify_all()
return result
def update(self, key, value):
with self.read_cond:
while self.reads:
self.read_cond.wait() # Wait for reads to clear
self.cache[key] = value # With read lock, update value
答案 0 :(得分:1)
您没有使用单一锁定。
您正在使用锁定和条件变量
self.read_lock = t.Condition(t.Lock())
条件变量也是并发原语。比锁更复杂的一个。
注意:请不要调用条件变量对象read_lock
修改强> 你的代码对我来说似乎是正确的,因为它解决了First readers-writers problem。正如你所说,它可能会使作家挨饿。这不是一个小问题。读者写作背后的逻辑是,可能会有比读写更多的读取 额外的锁定允许解决第二读者 - 作家问题,其中作家不会挨饿。实际上,当有作家等待资源时,读者必须等待。
答案 1 :(得分:0)
另一种使用锁和条件的解决方案。在解决饥饿问题的同时,还支持在同一线程请求时将读取锁升级为写入锁。
# From O'Reilly Python Cookbook by David Ascher, Alex Martelli
# With changes to cover the starvation situation where a continuous
# stream of readers may starve a writer, Lock Promotion and Context Managers
class ReadWriteLock:
""" A lock object that allows many simultaneous "read locks", but
only one "write lock." """
def __init__(self, withPromotion=False):
self._read_ready = threading.Condition(threading.RLock( ))
self._readers = 0
self._writers = 0
self._promote = withPromotion
self._readerList = [] # List of Reader thread IDs
self._writerList = [] # List of Writer thread IDs
def acquire_read(self):
logging.debug("RWL : acquire_read()")
""" Acquire a read lock. Blocks only if a thread has
acquired the write lock. """
self._read_ready.acquire( )
try:
while self._writers > 0:
self._read_ready.wait()
self._readers += 1
finally:
self._readerList.append(threading.get_ident())
self._read_ready.release( )
def release_read(self):
logging.debug("RWL : release_read()")
""" Release a read lock. """
self._read_ready.acquire( )
try:
self._readers -= 1
if not self._readers:
self._read_ready.notifyAll( )
finally:
self._readerList.remove(threading.get_ident())
self._read_ready.release( )
def acquire_write(self):
logging.debug("RWL : acquire_write()")
""" Acquire a write lock. Blocks until there are no
acquired read or write locks. """
self._read_ready.acquire( ) # A re-entrant lock lets a thread re-acquire the lock
self._writers += 1
self._writerList.append(threading.get_ident())
while self._readers > 0:
# promote to write lock, only if all the readers are trying to promote to writer
# If there are other reader threads, then wait till they complete reading
if self._promote and threading.get_ident() in self._readerList and set(self._readerList).issubset(set(self._writerList)):
break
else:
self._read_ready.wait( )
def release_write(self):
logging.debug("RWL : release_write()")
""" Release a write lock. """
self._writers -= 1
self._writerList.remove(threading.get_ident())
self._read_ready.notifyAll( )
self._read_ready.release( )
#----------------------------------------------------------------------------------------------------------
class ReadRWLock:
# Context Manager class for ReadWriteLock
def __init__(self, rwLock):
self.rwLock = rwLock
def __enter__(self):
self.rwLock.acquire_read()
return self # Not mandatory, but returning to be safe
def __exit__(self, exc_type, exc_value, traceback):
self.rwLock.release_read()
return False # Raise the exception, if exited due to an exception
#----------------------------------------------------------------------------------------------------------
class WriteRWLock:
# Context Manager class for ReadWriteLock
def __init__(self, rwLock):
self.rwLock = rwLock
def __enter__(self):
self.rwLock.acquire_write()
return self # Not mandatory, but returning to be safe
def __exit__(self, exc_type, exc_value, traceback):
self.rwLock.release_write()
return False # Raise the exception, if exited due to an exception
#----------------------------------------------------------------------------------------------------------