死锁现象

所谓死锁： 是指两个或两个以上的进程或线程在执行过程中，因争夺资源而造成的一种互相等待的现象，若无外力作用，它们都将无法推进下去。

此时称系统处于死锁状态或系统产生了死锁，这些永远在互相等待的进程称为死锁进程，如下就是死锁

from threading import Thread,Lock

import time

mutexA = Lock()

mutexB = Lock()

class MyThread(Thread):

def run(self):

self.task1()

self.task2()

def task1(self):

mutexA.acquire()

print('%s task1 get A' %self.name)

mutexB.acquire()

print('%s task1 get B' % self.name)

mutexB.release()

mutexA.release()

def task2(self):

mutexB.acquire()

print('%s task2 get B' % self.name)

time.sleep(1) # Thread-2 拿到执行权，执行get A出现死锁，此时thread2需要B锁，而thread1占用，与此同时，thread1需要A锁，thread2占用

mutexA.acquire()

print('%s task2 get A' % self.name)

mutexA.release()

mutexB.release()

if __name__ == '__main__':

for i in range(10):

t = MyThread()

t.start()

-------------------输出

Thread-1 task1 get A

Thread-1 task1 get B

Thread-1 task2 get B

Thread-2 task1 get A # 出现死锁，整个程序被阻塞

递归锁

解决方法，递归锁，在Python中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RLock。

这个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，二者的区别是：递归锁可以连续acquire多次，而互斥锁只能acquire一次

from threading import Thread,RLock

import time

mutexA = mutexB = RLock()

class MyThread(Thread):

def run(self):

self.task1()

self.task2()

def task1(self):

mutexA.acquire()

print('%s task1 get A' %self.name)

mutexB.acquire()

print('%s task1 get B' % self.name)

mutexB.release()

mutexA.release()

time.sleep(1) # Thread-2 拿到执行权，,此时counter=0，thread2执行task1

def task2(self):

mutexB.acquire()

print('%s task2 get B' % self.name)

mutexA.acquire()

print('%s task2 get A' % self.name)

mutexA.release()

mutexB.release()

if __name__ == '__main__':

for i in range(10):

t = MyThread()

t.start()

------------------------输出

Thread-1 task1 get A

Thread-1 task1 get B

Thread-2 task1 get A

Thread-2 task1 get B

Thread-3 task1 get A

Thread-3 task1 get B

Thread-4 task1 get A

Thread-4 task1 get B

Thread-5 task1 get A

Thread-5 task1 get B

Thread-6 task1 get A

Thread-6 task1 get B

Thread-7 task1 get A

Thread-7 task1 get B

Thread-8 task1 get A

Thread-8 task1 get B

Thread-9 task1 get A

Thread-9 task1 get B

Thread-10 task1 get A

Thread-10 task1 get B

Thread-1 task2 get B

Thread-1 task2 get A

Thread-2 task2 get B

Thread-2 task2 get A

Thread-4 task2 get B

Thread-4 task2 get A

Thread-3 task2 get B

Thread-3 task2 get A

Thread-5 task2 get B

Thread-5 task2 get A

Thread-6 task2 get B

Thread-6 task2 get A

……