Re: Threads and multiple interpreters

On Mon, Mar 5, 2018 at 9:41 PM, Jeff Allen <[email protected]> wrote:

> Although this is a compelling case for the JVM, it may not seem so for
> CPython. However, the same surely applies wherever a C extension loses
> interpreter context for an object that transfers between threads. In fact,
> such loss of context happens all the time when interpreting CPython byte
> code and has to be found each time we create a PyFrame, for example, by a
> call to PyThreadState_GET(), but hardly ever has the thread changed. Even
> for the JVM we should not think this applies only for objects explicitly
> created to represent tasks: any call-back, in fact, any invocation of a
> Python-defined operation (like PyObject._add, when it leads to the dunder
> add method) needs the same consideration if objects are able to be handled
> by different threads.
>
> At first, it seems as if every PyObject must have a reference to its
> owning interpreter. I think this is not the case. Since it is only the
> execution of code that raises the issue, and a (Python) function or method
> seems always able to find the module it belongs to, I expect only module
> objects to need this reference. The places where we must resolve the
> interpreter are those where we currently pick up the thread or interpreter
> state by a call to runtime support. Subject to a deeper look, I think the
> slow path via the module is only needed where presently there is no
> ThreadState and we fall back on the "default interpreter". But in the
> absence of certainty, we shouldn't have to guess.
>
>
Hi Jeff,

Having just read this, a potential counterexample came to my mind.

Suppose we want to pass a PyObject Pyo from PyInterpreterState PySa to
PyInterpreterState PySb. I'm not familiar with Jythons internals, but I
would assume many of the things that apply to passing object between
classloaders would apply here as well.
If the Pyo was loaded in a common classloader, the module of Pyo would
likely be loaded by that common classloader and the module would then be
unusable for distinguishing PySa and PySb.

Disclaimer: This is with very limited knowledge of Jython.

-- 
Eero Aaltonen

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