为什么多线程不会结束呀?

import threading
bb= 1000000
shu = 0
def jia (bb):
global shu
for x in range(bb):
shu += 1
if __name__ == '__main__':
t = threading.Thread(target=jia,args=(bb,))
t.start()
t.join()
print(shu)
哪位大神指教一下???结果出来了,,进程一直卡住不会结束.