[subinterpreters] Don't share Python objects between interpreters

[vstinner]

BPO	40533
Nosy	@vstinner, @ericsnowcurrently, @shihai1991
PRs	[WIP] bpo-40533: Make PyObject.ob_refcnt atomic in subinterpreters #19958 bpo-40533: Disable GC in subinterpreters #19961

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-05-06.15:11:35.602>
labels = ['expert-subinterpreters', '3.9']
title = "[subinterpreters] Don't share Python objects between interpreters"
updated_at = <Date 2022-01-13.17:32:56.034>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2022-01-13.17:32:56.034>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Subinterpreters']
creation = <Date 2020-05-06.15:11:35.602>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 40533
keywords = ['patch']
message_count = 8.0
messages = ['368262', '368274', '368276', '368279', '380105', '388966', '408606', '410499']
nosy_count = 3.0
nosy_names = ['vstinner', 'eric.snow', 'shihai1991']
pr_nums = ['19958', '19961']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue40533'
versions = ['Python 3.9']

[vstinner]

To get one GIL per interpreter (bpo-40512), either PyObject.ob_refcnt member must become an atomic variable, or subinterpreters must not share any object.

Right now, subinterpreters share Python objects. For example, PyModule_Type is declared statically and so shared by all interpreters and so PyModule_Type.tp_mro tuple is accessed in parallel by multiple interpreters. If PyObject.ob_refcnt is not atomic, Py_INCREF() and Py_DECREF() are unsafe and tp_mro tuple can be destroyed whereas it is still used.

I propose to make PyObject.ob_refcnt atomic for now, when Python is built with EXPERIMENTAL_ISOLATED_SUBINTERPRETERS macro defined. It's a temporary workaround until subinterpreters stop sharing objects.

[ericsnowcurrently]

Yep, before per-interpreter GIL is official we must get to the point where *no* PyObject objects are shared.

Making PyObject.ob_refcnt atomic until then (only as part of the experiment) should be fine.

[vstinner]

Yep, before per-interpreter GIL is official we must get to the point where *no* PyObject objects are shared.

I would like to add: "no PyObject objects are shared in the stdlib" ;-)

[vstinner]

New changeset d8135e9 by Victor Stinner in branch 'master':
bpo-40533: Disable GC in subinterpreters (GH-19961)
d8135e9

[vstinner]

For example, PyModule_Type is declared statically and so shared by all interpreters and so PyModule_Type.tp_mro tuple is accessed in parallel by multiple interpreters.

Another example of this issue are parallel Py_INCREF/Py_DECREF calls on the tp_bases member of a type.

[vstinner]

To get one GIL per interpreter (bpo-40512), either PyObject.ob_refcnt member must become an atomic variable, or subinterpreters must not share any object.

The current plan is to not share any object between two interpreters, so this PR is not needed. I close my PR 19958 which marked PyObject.ob_refcnt as atomic.

[vstinner]

Until all Python stdlib C extensions and all third-party C extensions will be modified to no longer use and define static types and will stop shared Python objects between two interpreters, we can no longer micro-optimize the comparison of two interned strings by only comparing their memory addresse. See bpo-46006.

[vstinner]

Sharing objects between multiple interpreters can cause complex bugs leading to crashes: https://bugs.python.org/issue46070#msg410493 For this specific bug, I wrote a workaround (GH-30577).

[vstinner]

I don't have the bandwidth to work on this issue, so I just close it.