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GeezerVison is showing github.com — gh-158194: Pin the type lock in PyType_Freeze() by nascheme · Pull Request #158674 · python/cpython

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@nascheme

@nascheme nascheme commented Oct 3, 2026 •
edited by bedevere-app Bot
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Fix PyType_Freeze releasing the type lock while waiting for a stop-the-world pause in the free-threaded build. Another thread could modify the type before the frozen type's caches were invalidated, and the release could deadlock against a thread waiting for the pause with the type lock held.

  • Issue: Free-threading: PyType_Freeze() can deadlock with concurrent class updates #158194

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@nascheme
pythongh-158194: Pin the type lock in PyType_Freeze()
e24318f 
PyType_Freeze() stopped the world under the type lock without pinning
it, so the lock could be released while waiting for the pause.  Pin it
with type_lock_prevent_release() like the other type-lock callers of
types_stop_world().
@nascheme
nascheme requested a review from markshannon as a code owner October 3, 2026 18:55
@nascheme nascheme added type-bug An unexpected behavior, bug, or error topic-free-threading needs backport to 3.14 bugs and security fixes needs backport to 3.15 bugs and security fixes labels Oct 3, 2026
@bedevere-app bedevere-app Bot mentioned this pull request Oct 3, 2026
Free-threading: PyType_Freeze() can deadlock with concurrent class updates #158194
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@bedevere-app bedevere-app Bot added the awaiting core review label Oct 3, 2026
@nascheme

nascheme commented Oct 3, 2026

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This fix in this PR is the same code change as suggested by @scasella, without the additional code comment.
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@johng

johng commented Oct 22, 2026 •
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Is it possible to add a test that exposes the original issue being fixed?

(I can see this might not be consistently reproducible though, not sure what the precedence is for that for)
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@nascheme

nascheme commented Oct 22, 2026

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Is it possible to add a test that exposes the original issue being fixed?

Yeah, that would be a good idea. I would guess it's possible.
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@nascheme

nascheme commented Oct 22, 2026

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This probably needs a re-write, given GH-159972.
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@nascheme

nascheme commented Nov 9, 2026

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Closing in favour of GH-161655
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@nascheme nascheme closed this Nov 9, 2026
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@markshannon markshannon Awaiting requested review from markshannon markshannon is a code owner

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awaiting core review needs backport to 3.14 bugs and security fixes needs backport to 3.15 bugs and security fixes topic-free-threading type-bug An unexpected behavior, bug, or error

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@nascheme @johng
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