python 死锁

import threading ,time

"""

当两个或者更多的线程在等待资源的时候就会产生死锁，两个线程相互等待。

在这里 thread1 等待thread2释放block , thread2等待thtead1释放ablock，

 

 

避免死锁的原则：

1. 一定要以一个固定的顺序来取得锁，这个列子中，意味着首先要取得alock, 然后再去block

2. 一定要按照与取得锁相反的顺序释放锁，这里，应该先释放block,然后是alock

"""

a = 5

 

alock = threading.Lock()

 

b = 5

 

block = threading.Lock()

 

 

def thread1calc():

    print "thread1 acquiring lock a"

    alock.acquire()

    time.sleep(5)

 

    print "thread1 acquiring lock b"

    block.acquire()

    a+=5

    b+=5

 

    print "thread1 releasing both locks"

    block.release()

    alock.release()

 

 

def thread2calc():

    print "thread2 acquiring lock b"

    block.acquire()

    time.sleep(5)

 

    print "thread2 acquiring lock a"

    alock.acquire()

    time.sleep(5)

    a+=10

    b+=10

 

    print "thread2 releasing both locks"

    block.release()

    alock.release()

 

 

t = threading.Thread(target = thread1calc)

t.setDaemon(1)

t.start()

 

t = threading.Thread(target = thread2calc)

t.setDaemon(2)

t.start()

 

while 1:

    time.sleep(300)

 

 

输出：

thread1 acquiring lock a

thread2 acquiring lock b

thread1 acquiring lock b

thread2 acquiring lock a