[Python-Dev] GIL vs thread state

[Guido van Rossum]

> The docs for PyThreadState_Clear() state that the interpreter lock must
> be held.
> 
> I had this code in ctypes to delete the thread state and release the lock:
> 
> static void LeavePython(char *msg)
> {
> 	PyThreadState *pts = PyThreadState_Swap(NULL);
> 	if (!pts)
> 		Py_FatalError("wincall (LeavePython): ThreadState is NULL?");
> 	PyThreadState_Clear(pts);
> 	PyThreadState_Delete(pts);
> 	PyEval_ReleaseLock();
> }
> 
> and (under certain coditions, when ptr->frame was not NULL), got

What is ptr->frame?  A typo for pts->frame?

If pts->frame is not NULL, I'd expect a warning from
PyThreadState_Clear(): "PyThreadState_Clear: warning: thread still has
a frame\n".

> "Fatal Python error: PyThreadState_Get: no current thread" in the call
> to PyThreadState_Clear().

That's strange, because I cannot trace the code in there to such a
call.  (Unless it is in a destructor.  Can you tell more about where
the PyThreadState_Get() call was?)

> The GIL is held while this code is executed, although there is no thread
> state. Changing the code to the following fixes the problem, it seems
> holding the GIL is not enough:
> 
> static void LeavePython(char *msg)
> {
> 	PyThreadState *pts = PyThreadState_Get();
> 	if (!pts)
> 		Py_FatalError("wincall (LeavePython): ThreadState is NULL?");
> 	PyThreadState_Clear(pts);
> 	pts = PyThreadState_Swap(NULL);
> 	PyThreadState_Delete(pts);
> 	PyEval_ReleaseLock();
> }
> 
> Is this a documentation problem, or a misunderstanding on my side?
> And, while we're on it, does the second version look ok?
> 
> Thomas
> 
> 
> _______________________________________________
> Python-Dev mailing list
> Python-Dev@python.org
> http://mail.python.org/mailman/listinfo/python-dev