[Python-Dev] pystate.c changes for Python 2.4.2 (original) (raw)

Tim Peters tim.peters at gmail.com
Fri Jan 20 17:08:08 CET 2006

[Tim]

... As a result, it so happens that core Python never uses the original PyThreadStateDelete() anymore, except when PyNewInterpreter() has to throw away the brand new thread state it created because it turns out it can't create a new interpreter.

[Michael]

Um, PyThreadStateDelete() is called from zapthreads() is called from PyInterpreterStateDelete() is called from PyFinalize()... so I don't entirely believe you here :)

That's only because I was wrong ;-) Thanks! That use was in fact half the cause of Python's segfaulting shutdown races, so it was exceptionally stupid of me to miss that one.

Since core Python never calls PyNewInterpreter() or PyThreadStateDelete(), you're in an intersection of areas no current Python developer even thinks about, let alone tests. But by the same token, because it's unused, there's nothing you can do to PyThreadStateDelete() that can hurt core Python either. That's why people should be more encouraging here ;-) It certainly makes sense to me that if a thread state is going away, any record of it in the auto-GIL-state machinery must also go away.

I guess if the patch fixes the original problem, it should go in -- my uneasiness stems not from worrying that it might do harm, but rather from the fact that I think it's incomplete. Maybe I just haven't thought it through enough.

Oh, I don't know whether it's "complete". I don't really understand multiple interpreters, have no idea why the OP is calling PyThreadState_Delete(), and fully expect that nobody is going to add tests to Python to check any of this. But Gabriel showed real skill at tracking down problems in this area, so if he creates new problems for himself by doing this (who else plays in this intersection? nobody I know of), I'm sure he'll figure them out ;-)

For example, it seems possible that he'll hit the same kinds of segfaulting shutdown races the Python core used to suffer. If a thread T releases the GIL, and goes on to delete its own thread state via PyThreadState_Delete(T's_thread_state), that is a race with Python shutdown's zapthreads trying to delete the same thing. That's why PyThreadState_DeleteCurrent(void) was introduced, and any call to PyThreadState_Delete() is suspect on this count.

Also, every time I look at pystate.c, I think of things that could be done differently or better -- for example, I think it would be sane and possibly even sensible to only call PyThreadsetkeyvalue() in PyGILStateNoteThreadState() if "tstate->interp == autoInterpreterState".

I joined Mark Hammond in ignoring the possibility of multiple interpreters when this stuff went in. The best thing to do with untested gimmicks without an audience willing to work on them is to remove them. If you want to "rehabilitate" multiple interpreters here, the best first step would be to write an addendum to PEP 311 documenting an intended design. The GIL state API is documented well enough on its own, but the intended semantics when mixing threads with multiple interpreters is clear as mud. PEP 311 asked "but who cares?", and nobody answered.

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