From: http://www.cnblogs.com/holbrook/archive/2012/03/08/2385449.html

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在線程間共享多個資源的時候，如果兩個線程分別占有一部分資源并且同時等待對方的資源，就會造成死鎖。盡管死鎖很少發生，但一旦發生就會造成應用的停止響應。下面看一個死鎖的例子：

# 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()

執行結果：

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

此時進程已經死掉。

可重入鎖

更簡單的死鎖情況是一個線程“迭代”請求同一個資源，直接就會造成死鎖：

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()

為了支持在同一線程中多次請求同一資源，python提供了“可重入鎖”：threading.RLock。RLock內部維護著一個Lock和一個counter變量，counter記錄了acquire的次數，從而使得資源可以被多次require。直到一個線程所有的acquire都被release，其他的線程才能獲得資源。上面的例子如果使用RLock代替Lock，則不會發生死鎖：

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()

執行結果：

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

總結

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