[Python-Dev] Re: [Stackless] comments on PEP 219

[Christian Tismer]

Greg Ewing wrote:
> 
> Christian Tismer <tismer@tismer.com>:
> 
> > It does not
> > matter how and where frames were created, it is just impossible
> > to jump at a frame that is held by an interpreter on the C stack.
> 
> I think I need a clearer idea of what it means for a frame
> to be "held by an interpreter".
> 
> I gather that each frame has a lock flag. How and when does
> this flag get set and cleared?

Assume a frame F being executed by an interpreter A.
Now, if this frame calls a function, which in turn
starts another interpreter B, this hides interpreter
A on the C stack. Frame F cannot be run by anything
until interpreter B is finished.
Exactly in this situation, frame F has its lock set,
to prevend crashes.
Such a locked frame cannot be a switch target.

ciao - chris

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