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
