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[subinterpreters] Don't share Python objects between interpreters #84713

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vstinner opened this issue May 6, 2020 · 9 comments
Closed

[subinterpreters] Don't share Python objects between interpreters #84713

vstinner opened this issue May 6, 2020 · 9 comments
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3.9 only security fixes topic-subinterpreters

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@vstinner
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vstinner commented May 6, 2020

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
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    vstinner commented May 6, 2020

    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.

    @vstinner vstinner added interpreter-core (Objects, Python, Grammar, and Parser dirs) 3.9 only security fixes labels May 6, 2020
    @ericsnowcurrently
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    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
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    vstinner commented May 6, 2020

    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
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    vstinner commented May 6, 2020

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

    @vstinner vstinner added topic-subinterpreters and removed interpreter-core (Objects, Python, Grammar, and Parser dirs) labels May 15, 2020
    @vstinner vstinner changed the title Subinterpreters: don't share Python objects between interpreters [subinterpreters] Don't share Python objects between interpreters May 15, 2020
    @vstinner vstinner added topic-subinterpreters and removed interpreter-core (Objects, Python, Grammar, and Parser dirs) labels May 15, 2020
    @vstinner vstinner changed the title Subinterpreters: don't share Python objects between interpreters [subinterpreters] Don't share Python objects between interpreters May 15, 2020
    @vstinner
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    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
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    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
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    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
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    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).

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
    @vstinner
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    vstinner commented Nov 3, 2022

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

    @vstinner vstinner closed this as completed Nov 3, 2022
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