[Python-Dev] Extension modules, Threading, and the GIL

[Martin v. Löwis]

David Abrahams <dave@boost-consulting.com> writes:

> There was no deadlock, as I've said.  

Yes, you only said this was a no-no, in

http://mail.python.org/pipermail/python-dev/2002-December/031449.html

I inferred from that (apparently incorrectly) that the no-no is a
deadlock.

> The symptom is that Python complains at some point that there's no
> thread state.  It goes away if A releases the GIL before calling
> into Qt, and reacquires the GIL afterwards.

Now I'm confused. In

http://mail.python.org/pipermail/python-dev/2002-December/031424.html

you said "A must also release the GIL" and then "the author of A may
have had no reason to believe anyone would install Python callbacks in
Q". From that I inferred that A does *not* release the GIL (as the
author had no reason to).

Now you are saying that A releases the GIL. Which one it is?

> I speculate that the callback releases the GIL when it is finished,
> so that when A returns to Python there is no current thread.

That would be a bug in the callback. If there was a thread state when
it was called, there should be a thread state when it returns.

> Hmm, it seems as though a mutex-protected record of which thread is
> currently holding the GIL should be enough to handle it. 

It depends on what "it" is, here. This one?

Q: Is there a way to find out whether the current thread holds the GIL?

If so, a mutex-protected record might work, but also might be
expensive.

Regards,
Martin