multiprocessing.Event.set() can deadlock when called from a signal handler or trace function

[ivarref]

Bug report

Bug description:

multiprocessing.Event.set() will acquire a lock when setting the internal flag. multiprocessing.Event.is_set() will acquire the same lock when checking the flag. Thus if a signal handler calls .set() when .is_set() is running on the same process, there will be a deadlock.

multiprocessing.Event uses a regular non-reentrantlock lock. This should be changed to a reentrant lock. Please see the pull request.

Thanks for all the work on the Python programming language. I appreciate all your efforts highly.

Kind regards.

Example program below that (sometimes) deadlocks. On my machine I typically need to run it less than 10 times before a deadlock occurs. Also included in the code block is a sample stacktrace.

import faulthandler
import multiprocessing
import os
import signal


def run_buggy():
    shutdown_event = multiprocessing.Event()

    def sigterm_handler(_signo, _stack_frame):
        try:
            print(f'sigterm_handler running')
            shutdown_event.set()
        finally:
            print(f'sigterm_handler done')

    signal.signal(signal.SIGTERM, sigterm_handler)
    signal.signal(signal.SIGINT, sigterm_handler)
    faulthandler.register(signal.SIGUSR1)

    print(f'Running process with PID {os.getpid()}')
    print(f'Dump the stack by executing:')
    print(f'kill -SIGUSR1 {os.getpid()}')
    print(f'Try to kill this process with CTRL-C or kill {os.getpid()}')
    while not shutdown_event.is_set():
        pass

if __name__ == '__main__':
    run_buggy()
# Sample stacktrace:
# Current thread 0x00000001ed74cf40 (most recent call first):
#   File "/Users/ire/code/cpython/Lib/multiprocessing/synchronize.py", line 95 in __enter__
#   File "/Users/ire/code/cpython/Lib/multiprocessing/synchronize.py", line 237 in __enter__
#   File "/Users/ire/code/cpython/Lib/multiprocessing/synchronize.py", line 342 in set
#   File "/Users/ire/code/cpython/./bug.py", line 13 in sigterm_handler
#   File "/Users/ire/code/cpython/Lib/multiprocessing/synchronize.py", line 95 in __enter__
#   File "/Users/ire/code/cpython/Lib/multiprocessing/synchronize.py", line 237 in __enter__
#   File "/Users/ire/code/cpython/Lib/multiprocessing/synchronize.py", line 335 in is_set
#   File "/Users/ire/code/cpython/./bug.py", line 25 in run_buggy
#   File "/Users/ire/code/cpython/./bug.py", line 29 in <module>

CPython versions tested on:

3.11, 3.12, CPython main branch

Operating systems tested on:

Linux, macOS

Linked PRs

• gh-126434: Use multiprocessing.Value for multiprocessing.Event to avoid deadlock when there is reentrant usage of set from is_set, e.g. when handling system signals #126437

[Zheaoli]

Bug confirmed on main branch

[Zheaoli]

@Eclips4 We need tag for this issue, I think topic-multiprocessing would be OK

BTW @ivarref are you already working on a PR?

[ivarref]

Hi @Zheaoli

Thanks for the quick response. I've pushed a pull request now. What do you think?

Thanks and kind regards.

[ivarref]

Update: I've added a blurb entry.

[colesbury]

I commented on the PR, but I don't think multiprocessing.Event is safe to call from a Python signal handler, even with an RLock. I suspect that's true of a lot of Python's concurrency primitives.

The general problem is that Python can execute signal handling code asynchronously at many places that don't expect to support reentrancy.

RLock's don't solve this problem because the things they protect are often not safe to call reentrancy. In other words, calling them reentrantly breaks invariants that a (non-reentrant) lock would otherwise protect.