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gh-144295: Fix data race in dict method lookup and global load#144312

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colesbury merged 1 commit intopython:mainfrom
colesbury:gh-144295-data-race
Jan 30, 2026
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gh-144295: Fix data race in dict method lookup and global load#144312
colesbury merged 1 commit intopython:mainfrom
colesbury:gh-144295-data-race

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@colesbury colesbury commented Jan 28, 2026 •
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In _PyDict_GetMethodStackRef, only use the fast-path unicode lookup when the dict is owned by the current thread or already marked as shared. This prevents a race between the lookup and concurrent dict resizes, which may free the PyDictKeysObject (i.e., it ensures that the resize uses QSBR).

Address a similar issue in _Py_dict_lookup_threadsafe_stackref by calling ensure_shared_on_read().

@colesbury
pythongh-144295: Fix data race in dict method lookup and global load
af80f57 
In `_PyDict_GetMethodStackRef`, only use the fast-path unicode lookup
when the dict is owned by the current thread or already marked as shared.
This prevents a race between the lookup and concurrent dict resizes,
which may free the PyDictKeysObject (i.e., it ensures that the resize
uses QSBR).

Address a similar issue in `_Py_dict_lookup_threadsafe_stackref` by
calling `ensure_shared_on_read()`.
@bedevere-app bedevere-app bot mentioned this pull request Jan 28, 2026
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@colesbury colesbury marked this pull request as ready for review January 29, 2026 12:25
@colesbury colesbury merged commit e666a01 into python:main Jan 30, 2026
91 of 93 checks passed
@colesbury colesbury deleted the gh-144295-data-race branch January 30, 2026 16:14
@colesbury colesbury added the needs backport to 3.14 bugs and security fixes label Jan 30, 2026
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miss-islington-app bot commented Jan 30, 2026

Thanks @colesbury for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

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miss-islington-app bot commented Jan 30, 2026

Sorry, @colesbury, I could not cleanly backport this to 3.14 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker e666a01ef42939f77f4c22ca47a610df5ef8b7ab 3.14

colesbury added a commit to colesbury/cpython that referenced this pull request Jan 30, 2026
@colesbury
[3.14] pythongh-144295: Fix data race in dict method lookup and globa…
aba5e3d 
…l load (pythongh-144312)

In `_PyDict_GetMethodStackRef`, only use the fast-path unicode lookup
when the dict is owned by the current thread or already marked as shared.
This prevents a race between the lookup and concurrent dict resizes,
which may free the PyDictKeysObject (i.e., it ensures that the resize
uses QSBR).

Address a similar issue in `_Py_dict_lookup_threadsafe_stackref` by
calling `ensure_shared_on_read()`.
(cherry picked from commit e666a01)

Co-authored-by: Sam Gross <colesbury@gmail.com>
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bedevere-app bot commented Jan 30, 2026

GH-144346 is a backport of this pull request to the 3.14 branch.

@bedevere-app bedevere-app bot removed the needs backport to 3.14 bugs and security fixes label Jan 30, 2026
colesbury added a commit that referenced this pull request Jan 30, 2026
@colesbury
[3.14] gh-144295: Fix data race in dict method lookup and global load (…
06f9c8c 
…gh-144312) (#144346)

In `_Py_dict_lookup_threadsafe_stackref`, call `ensure_shared_on_read()` to
prevent a race between the lookup and concurrent dict resizes, which may free
the PyDictKeysObject (i.e., it ensures that the resize uses QSBR).

(cherry picked from commit e666a01)
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bedevere-bot commented Jan 30, 2026

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot x86 Debian Installed with X 3.14 (no tier) has failed when building commit 06f9c8c.

What do you need to do:

  1. Don't panic.
  2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
  3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1793/builds/902) and take a look at the build logs.
  4. Check if the failure is related to this commit (06f9c8c) or if it is a false positive.
  5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1793/builds/902

Summary of the results of the build (if available):

==

Click to see traceback logs 
Traceback (most recent call last):
  File �[35m"/buildbot/buildarea/3.14.ware-debian-x86.installed/build/target/lib/python3.14/test/test_annotationlib.py"�[0m, line �[35m161�[0m, in �[35mtest_nonexistent_attribute�[0m
    �[31mself.assertEqual�[0m�[1;31m(epsilon_anno, support.EqualToForwardRef("some | {obj, module}", owner=f))�[0m
    �[31m~~~~~~~~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^�[0m
�[1;35mAssertionError�[0m: �[35mForwardRef('some | {module, obj}', owner=[79 chars]1e0>) != EqualToForwardRef('some | {obj, module}',[86 chars]1e0>)�[0m


Traceback (most recent call last):
  File �[35m"/buildbot/buildarea/3.14.ware-debian-x86.installed/build/target/lib/python3.14/test/test_annotationlib.py"�[0m, line �[35m161�[0m, in �[35mtest_nonexistent_attribute�[0m
    �[31mself.assertEqual�[0m�[1;31m(epsilon_anno, support.EqualToForwardRef("some | {obj, module}", owner=f))�[0m
    �[31m~~~~~~~~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^�[0m
�[1;35mAssertionError�[0m: �[35mForwardRef('some | {module, obj}', owner=[79 chars]868>) != EqualToForwardRef('some | {obj, module}',[86 chars]868>)�[0m

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@mpage mpage mpage approved these changes

@methane methane Awaiting requested review from methane methane is a code owner

@markshannon markshannon Awaiting requested review from markshannon markshannon is a code owner

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