python多线程之死锁现象与递归锁

python多线程之死锁现象与递归锁

什么叫死锁现象？当两个线程相互等待对方释放资源时，就会发生死锁。
比如：

from threading import Thread
from threading import Lock
import time

lock_A = Lock()
lock_B = Lock()

class MyThread(Thread):
    def run(self):
        self.f1()
        self.f2()

    def f1(self):
        lock_A.acquire()
        print(f"{self.name}拿到了A锁")

        lock_B.acquire()
        print(f"{self.name}拿到了B锁")

        lock_B.release()
        print(f"{self.name}释放了B锁")
        lock_A.release()
        print(f"{self.name}释放了A锁")

    def f2(self):
        lock_B.acquire()
        print(f"{self.name}拿到了B锁")
        time.sleep(0.1)

        lock_A.acquire()
        print(f"{self.name}拿到了A锁")

        lock_A.release()
        print(f"{self.name}释放了A锁")
        lock_B.release()
        print(f"{self.name}释放了B锁")

if __name__ == "__main__":
    for i in range(3):
        t = MyThread()
        t.start()

Thread-1拿到了A锁
Thread-1拿到了B锁
Thread-1释放了B锁
Thread-1释放了A锁
Thread-1拿到了B锁
Thread-2拿到了A锁

thread1 先抢到了A锁，此时t2 t3也想抢A锁，但只能等待，等待t1释放A锁，t1又抢了B锁，此时t1有A B两把锁没有释放 t2 t3 继续等待；当t1依此释放B，A锁时，线程t2，t3 争抢A锁，按照结果分析，t1抢到B锁，t2抢到了A锁，t1拿到了B锁，但是接下来，t1睡了0.1秒，t1拥有着B锁，想要A锁，t2拥有着A锁，想要B锁，这就形成了死锁现象。

递归锁可以解决死锁现象，业务需要多个锁时，先要考虑递归锁：

from threading import Thread
from threading import RLock
import time

lock_A = lock_B = RLock()  # 必须这样写

# 这样写不行
# lock_A = RLock
# lock_B = lock_A

# 递归锁有一个计数的功能：原数字为0，上一次锁，计数+1，释放一次锁，计数-1
# 只要递归锁上面的数字不为0，其他线程就不能抢锁
class MyThread(Thread):
    def run(self):
        self.f1()
        self.f2()

    def f1(self):
        lock_A.acquire()
        print(f"{self.name}拿到了A锁")

        lock_B.acquire()
        print(f"{self.name}拿到了B锁")

        lock_B.release()
        print(f"{self.name}释放了B锁")
        lock_A.release()
        print(f"{self.name}释放了A锁")

    def f2(self):
        lock_B.acquire()
        print(f"{self.name}拿到了B锁")
        time.sleep(0.1)

        lock_A.acquire()
        print(f"{self.name}拿到了A锁")

        lock_A.release()
        print(f"{self.name}释放了A锁")
        lock_B.release()
        print(f"{self.name}释放了B锁")

if __name__ == "__main__":
    for i in range(3):
        t = MyThread()
        t.start()

Thread-1拿到了A锁
Thread-1拿到了B锁
Thread-1释放了B锁
Thread-1释放了A锁
Thread-1拿到了B锁
Thread-1拿到了A锁
Thread-1释放了A锁
Thread-1释放了B锁
Thread-2拿到了A锁
Thread-2拿到了B锁
Thread-2释放了B锁
Thread-2释放了A锁
Thread-2拿到了B锁
Thread-2拿到了A锁
Thread-2释放了A锁
Thread-2释放了B锁
Thread-3拿到了A锁
Thread-3拿到了B锁
Thread-3释放了B锁
Thread-3释放了A锁
Thread-3拿到了B锁
Thread-3拿到了A锁
Thread-3释放了A锁
Thread-3释放了B锁

Process finished with exit code 0