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python多线程编程(4): 死锁和可重入锁

zihua 2014-01-20 23:01:54 点击: 624 | 收藏


死锁

在线程间共享多个资源的时候,如果两个线程分别占有一部分资源并且同时等待对方的资源,就会造成死锁。尽管死锁很少发生,但一旦发生就会造成应用的停止响应。下面看一个死锁的例子:

# encoding: UTF-8

import threading

import time

class MyThread(threading.Thread):

def do1(self):

global resA, resB

if mutexA.acquire():

msg = self.name+' got resA'

print msg

if mutexB.acquire(1):

msg = self.name+' got resB'

print msg

mutexB.release()

mutexA.release()

def do2(self):

global resA, resB

if mutexB.acquire():

msg = self.name+' got resB'

print msg

if mutexA.acquire(1):

msg = self.name+' got resA'

print msg

mutexA.release()

mutexB.release()

def run(self):

self.do1()

self.do2()

resA = 0

resB = 0

mutexA = threading.Lock()

mutexB = threading.Lock()

def test():

for i in range(5):

t = MyThread()

t.start()

if __name__ == '__main__':

test()

执行结果:

Thread-1 got resA

Thread-1 got resB

Thread-1 got resB

Thread-1 got resA

Thread-2 got resA

Thread-2 got resB

Thread-2 got resB

Thread-2 got resA

Thread-3 got resA

Thread-3 got resB

Thread-3 got resB

Thread-3 got resA

Thread-5 got resA

Thread-5 got resB

Thread-5 got resB

Thread-4 got resA

此时进程已经死掉。

可重入锁

更简单的死锁情况是一个线程“迭代”请求同一个资源,直接就会造成死锁:

import threading

import time

class MyThread(threading.Thread):

def run(self):

global num

time.sleep(1)

if mutex.acquire(1):

num = num+1

msg = self.name+' set num to '+str(num)

print msg

mutex.acquire()

mutex.release()

mutex.release()

num = 0

mutex = threading.Lock()

def test():

for i in range(5):

t = MyThread()

t.start()

if __name__ == '__main__':

test()

为了支持在同一线程中多次请求同一资源,python提供了“可重入锁”:threading.RLock。RLock内部维护着一个Lock和一个counter变量,counter记录了acquire的次数,从而使得资源可以被多次require。直到一个线程所有的acquire都被release,其他的线程才能获得资源。上面的例子如果使用RLock代替Lock,则不会发生死锁:

import threading

import time

class MyThread(threading.Thread):

def run(self):

global num

time.sleep(1)

if mutex.acquire(1):

num = num+1

msg = self.name+' set num to '+str(num)

print msg

mutex.acquire()

mutex.release()

mutex.release()

num = 0

mutex = threading.RLock()

def test():

for i in range(5):

t = MyThread()

t.start()

if __name__ == '__main__':

test()

原文链接:http://www.wumii.com/item/13cAiJlzg

作者:zihua | 分类: python | 标签: python | 阅读: 624 | 发布于: 2014-01-20 23时 |