Python3多进程子进程首次网络访问时偶发futex_wait_queue_me阻塞

问题现象 #

• 主程序基于Python3.9开发，通过multiprocessing模块加载并控制子进程执行短期任务，任务完成后子进程终止
• 部分子进程启动后能正常输出调试日志，但首次发起网络访问时会卡住：既可能是requests请求，也可能是pymssql数据库连接
• 卡住的子进程会永久处于futex_wait_queue_me状态，无法通过SIGTERM信号终止，必须使用SIGKILL才能强制停止
• 问题进程的栈信息如下：

cat /proc/12345/stack
 <0>] futex_wait_queue_me+0xb6/0x110
[<0>] futex_wait+0xe9/0x240
[<0>] do_futex+0x174/0xbc0
[<0>] __x64_sys_futex+0x146/0x1c0
[<0>] do_syscall_64+0x33/0x80
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

已尝试的无效方案 #

怀疑父进程继承的文件句柄携带锁，在子进程初始化时添加os.closerange(3,100)关闭除stdin/stdout/stderr外的所有句柄，但问题依旧。相关代码如下：

def startSub(params, config):
  sub=Process(target=SubModule, args=(params, config))
  sub.daemon=True
  sub.start()
  return sub
  # 子进程独立运行至终止，无IPC，仅通过sub.is_alive()监控状态

class SubModule:
  def __init__(self, params, config):
    os.closerange(3,100)
    setproctitle("Submodule")                    # 标识进程名称
    log.debug("Starting stuff")                  # 该日志能正常输出
    result=requests.get("https://somewhere.org") # 此处偶尔会卡住不返回
    log.debug("came back from requests")         # 卡住时不会执行到这里

GDB调试回溯信息 #

程序运行在python3.9-slim-bullseye容器中，添加gdb后捕获到的Python层回溯：

(gdb) py-bt
Traceback (most recent call first):
  <built-in method getaddrinfo of module object at remote 0x7f4fd9634350>
  File "/usr/lib/python3.9/socket.py", line 953, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
  File "/usr/lib/python3/dist-packages/urllib3/util/connection.py", line 329, in create_connection
  File "/usr/lib/python3/dist-packages/urllib3/connection.py", line 681, in _new_conn
  File "/usr/lib/python3/dist-packages/urllib3/connection.py", line 353, in connect
    conn = self._new_conn()
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 1012, in _validate_conn
    conn.connect()
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 382, in _make_request
    self._validate_conn(conn)
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 1467, in urlopen
  File "/usr/lib/python3/dist-packages/requests/adapters.py", line 951, in send
  File "/usr/lib/python3/dist-packages/requests/sessions.py", line 655, in send
    r = adapter.send(request, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/sessions.py", line 1310, in request
  File "/usr/lib/python3/dist-packages/requests/api.py", line 61, in request

底层C栈信息显示，程序卡在_socket.getaddrinfo(host, port, socket.AF_INET, socket.SOCK_STREAM, 0)调用内部：

(gdb) bt
#0  0x00007f4fd9c19d66 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007f4fd9cb4bed in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#2  0x00007f4fd9cb4fcb in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#3  0x00007f4fd9cb3955 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#4  0x00007f4fd9cb474b in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#5  0x00007f4fd9cb48d3 in __resolv_context_get () from /lib/x86_64-linux-gnu/libc.so.6
#6  0x00007f4fd9ca54e9 in gethostbyname2_r () from /lib/x86_64-linux-gnu/libc.so.6
#7  0x00007f4fd9c7cfac in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#8  0x00007f4fd9c7db25 in getaddrinfo () from /lib/x86_64-linux-gnu/libc.so.6
#9  0x0000000000615b1e in socket_getaddrinfo (self=<optimized out>, args=<optimized out>, kwargs=<optimized out>) at ../Modules/socketmodule.c:6574
#10 0x000000000065e4fd in cfunction_call (func=<built-in method getaddrinfo of module object at remote 0x7f4fd9634350>, args=<optimized out>, kwargs=<optimized out>)
    at ../Objects/methodobject.c:539

注：此问题并非常见的getaddrinfo调用锁死情况，而是卡在getaddrinfo内部的锁机制中。

目前毫无头绪，求解释这一现象？

内容的提问来源于stack exchange，提问作者Andreas