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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"PyUnstable_Code_SetExtra and PyUnstable_Code_GetExtra are not thread-safe in free-threaded builds","articleBody":"# Bug report\n\n### Bug description:\n\n`PyUnstable_Code_SetExtra` and `PyUnstable_Code_GetExtra` in `Objects/codeobject.c` have no free-threaded protections. The `co_extra` field is read and written with plain loads and stores, and `SetExtra` calls `PyMem_Realloc` which can free the old buffer while a concurrent `GetExtra` is reading from it.\n\nSimilarly, `PyUnstable_Eval_RequestCodeExtraIndex` in `Python/ceval.c` increments `co_extra_user_count` and writes to `co_extra_freefuncs` without any synchronization.\n\nThe new tests (#144980) exercise the concurrent paths and a free-threaded TSAN build without the fix crashes/emits warnings.\n\n\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nLinux\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-144980\n* gh-145052\n* gh-145327\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/yoney","@type":"Person","name":"yoney"},"datePublished":"2026-02-18T22:56:50.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/144981/cpython/issues/144981"}