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()
执行结果：
thread-1 set num to 1
thread-3 set num to 2
thread-2 set num to 3
thread-5 set num to 4
thread-4 set num to 5