python线程死锁与递归锁
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 ……