gh-155400: Fix deadlock with type_lock_prevent_release() - #155401
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kumaraditya303 merged 2 commits intoAug 15, 2026
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Two threads assigning to a special method of the same class could deadlock in the free-threaded build. If type_lock_prevent_release() only acts on TYPE_LOCK then we can get lock inversion. This happens when BEGIN_TYPE_DICT_LOCK() is used and the critical section holds both mutexes. We need to prevent release of both mutexes. Assisted-by: Claude
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This PR requires GH-155370 to be merged first otherwise the unit test added exposes data-races. |
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Setting as release blocker. When discussing this with @kumaraditya303, he suspects that some libraries could trigger this deadlock when they create types. |
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Thanks @nascheme for the PR, and @kumaraditya303 for merging it 🌮🎉.. I'm working now to backport this PR to: 3.15. |
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GH-155822 is a backport of this pull request to the 3.15 branch. |
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Note that a unit test added by this PR exposes an existing leak, see gh-155978. This leak causes some refleak build bots to fail. |
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Two threads assigning to a special method of the same class could deadlock in the free-threaded build. If type_lock_prevent_release() only acts on TYPE_LOCK then we can get lock inversion. This happens when BEGIN_TYPE_DICT_LOCK() is used and the critical section holds both mutexes. We need to prevent release of both mutexes.