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-8import threadingimport 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 = 0resB = 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 threadingimport 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 = 0mutex = 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 threadingimport 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 = 0mutex = 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
