[New-bugs-announce] [issue17969] multiprocessing crash on exit

Kristján Valur Jónsson report at bugs.python.org
Mon May 13 13:31:34 CEST 2013

New submission from Kristján Valur Jónsson:

We have observed this crash with some frequency when running our compilation scripts using multiprocessing.Pool()

By analysing the crashes, this is what is happening:
1) The Pool has a "daemon" thread managing the pool.
2) the worker is asleep, waiting for the GIL
3) The main thread exits.  The system starts its shutdown. During PyInterpreterState_Clear, it has cleared among other things the sys dict.  During this, it clears an old traceback.  The traceback contains a multiprocessing.connection object.
4) The connection object is cleared.  It it contains this code:
5) The sleeping daemon thread is woken up and starts prancing around.  Upon calling sys.exc_clear() it crashes, since the tstate->interp->sysdict == NULL.

I have a workaround in place in our codebase:

static void
connection_dealloc(ConnectionObject* self)
    if (self->weakreflist != NULL)

    if (self->handle != INVALID_HANDLE_VALUE) {
        /* CCP Change.  Cannot release threads here, because this
         * deallocation may be running during process shutdown, and
         * releaseing a daemon thread will cause a crash

In general, deallocators should have no side effects, I think.  Releaseing the GIL is certainly a side effect.

I realize that process shutdown is a delicate matter.  One delicate thing is that we cannot allow worker threads to run anymore.  I see no general mechanism for ensuring this, but surely at least not releasing the GIL for deallocators is a first step?

messages: 189123
nosy: kristjan.jonsson
priority: normal
severity: normal
status: open
title: multiprocessing crash on exit
type: crash
versions: Python 2.7

Python tracker <report at bugs.python.org>

More information about the New-bugs-announce mailing list