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gh-151593: Fix dead lock in PyDict insert_split_key() - #152200

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vstinner:dict_deadlock
Jun 25, 2026
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gh-151593: Fix dead lock in PyDict insert_split_key()#152200
vstinner merged 2 commits into
python:mainfrom
vstinner:dict_deadlock

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@vstinner

@vstinnervstinner commented Jun 25, 2026•
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Do not hold LOCK_KEYS() lock when calling PyType_Modified() to avoid a deadlock.

@vstinner@nascheme
pythongh-151593: Fix dead lock in PyDict insert_split_key()
bbc0359
Do not hold LOCK_KEYS() lock when calling PyType_Modified() to avoid
a deadlock.
Co-authored-by: Neil Schemenauer <nas-github@arctrix.com>
@bedevere-appbedevere-appBot mentioned this pull request Jun 25, 2026
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@vstinner

vstinner commented Jun 25, 2026

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On Python built with ./configure --with-pydebug --with-system-expat --with-thread-sanitizer --disable-gil, I can easily reproduce the issue with:

./python -m test test_abc --parallel-threads=10 -v -F

It only takes 1 or 2 iterations to trigger the deadlock.

With this change, I can no longer reproduce the deadlock: the test is running for 5 minutes (test_abc passed 35 times in a row).

@vstinnervstinner mentioned this pull request Jun 25, 2026
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@vstinner

vstinner commented Jun 25, 2026

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@dpdani@colesbury@nascheme: Would you mind to review this fix?

I just converted @nascheme's comment into an actual PR.

dpdani
dpdani reviewed Jun 25, 2026
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I'd add a comment, but otherwise LGTM 👍

Comment threadObjects/dictobject.c
@vstinner

vstinner commented Jun 25, 2026

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@dpdani: I added a comment on LOCK_KEYS() based on previous comments on pull requests and issues. Does it look good to you?

colesbury
colesbury approved these changes Jun 25, 2026
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Thanks for fixing this

@vstinner
vstinner merged commit bef5706 into python:mainJun 25, 2026
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@vstinner
vstinner deleted the dict_deadlock branch June 25, 2026 17:58
@vstinner

vstinner commented Jun 25, 2026

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Merged. Thanks for reviews!

This was referenced Jun 25, 2026
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nascheme added a commit that referenced this pull request Jun 26, 2026
@nascheme
gh-152238: Revertgh-150490andgh-152200. (gh-152232)
9626ef8
Revert commits:
gh-151593: Fix dead lock in PyDict insert_split_key() (#152200)
gh-150490: Raise PyType_Modified for insertion into split dictionary (#150489)
For gh-150489, it violates locking discipline and results in deadlocks,
gh-151593 is an example of it being hit in CI. The attempted fixgh-152200 avoids the deadlock but introduces a data-race. The race
window is small but can be triggered with pure Python code.
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@colesburycolesburycolesbury approved these changes
@methanemethaneAwaiting requested review from methanemethane is a code owner
@markshannonmarkshannonAwaiting requested review from markshannonmarkshannon is a code owner
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@vstinner@colesbury@dpdani