python 可重入锁有什么用?

Question

可重入锁:支持在同一线程中多次请求同一资源

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()
      print('Do another thing.')
      mutex.release()
      mutex.release()
num = 0
mutex = threading.RLock()
def test():
  for i in range(5):
    t = MyThread()
    t.start()
if __name__ == '__main__':
  test()

那我既然一个线程要用两次共享变量,我直接acquire后我就不release,干嘛要acquire两次不是多此一举?
直接只有一次acquire和一次release:

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)
      print('Do another thing.')
      mutex.release()
num = 0
mutex = threading.RLock()
def test():
  for i in range(5):
    t = MyThread()
    t.start()
if __name__ == '__main__':
  test()

还是我学艺不精,说可重入锁是用来防止死锁的,那么可重入锁应该用在什么情况下?

Answer 1

可重入锁不是这么用的，一般是在面向对象中使用，比如

class A:
   def f1(self):
       mutex.acquire()
       try:
           #do something
       finally:
           mutex.release()
   def f2(self):
       mutex.acquire()
       try:
           #do something
       finally:
           mutex.release()

def run1(obj):
    obj.f1()
    obj.f2()
    
def run2(obj):
    obj.f2()
    obj.f1()

obj1 = A()
t1 = threading.Thread(target=run1, args=(obj1, ))
t2 = threading.Thread(target=run2, args=(obj1, ))
t1.start()
t2.start()

调用顺序不同，而且都需要同步的时候，如果不用递归锁，会死锁。如果f1或f2不加锁，数据不同步，报错