René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Py_(X)SETREF should have atomic versions on free-threaded builds","articleBody":"# Feature or enhancement\n\n### Proposal:\n\nRight now, `Py_SETREF` is implemented in non-atomic terms, which makes it subject to operation reordering issues on weakly ordered architectures (which caused a problem fixing gh-149816, where even switching from separate `Py_CLEAR` and later assignment to `set_sni_cb` to a single \"atomic\" `Py_SETREF` didn't fix the issue on ARM, because SSL's callback was being invoked asynchronously).\n\n`Py_SETREF` is naturally implemented in terms of an atomic exchange, and at least for the free-threaded builds, it seems like it should be implemented as such to avoid such raciness, supplementing the existing:\n\n```c\n#ifdef _Py_TYPEOF\n#define Py_SETREF(dst, src) \\\n    do { \\\n        _Py_TYPEOF(dst)* _tmp_dst_ptr = \u0026(dst); \\\n        _Py_TYPEOF(dst) _tmp_old_dst = (*_tmp_dst_ptr); \\\n        *_tmp_dst_ptr = (src); \\\n        Py_DECREF(_tmp_old_dst); \\\n    } while (0)\n#else\n#define Py_SETREF(dst, src) \\\n    do { \\\n        PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, \u0026(dst)); \\\n        PyObject *_tmp_old_dst = (*_tmp_dst_ptr); \\\n        PyObject *_tmp_src = _PyObject_CAST(src); \\\n        memcpy(_tmp_dst_ptr, \u0026_tmp_src, sizeof(PyObject*)); \\\n        Py_DECREF(_tmp_old_dst); \\\n    } while (0)\n#endif\n\n// Similarly long nonsense for Py_XSETREF\n```\n\nwith a new atomic-based approach for free-threaded builds:\n\n```c\n#define Py_SETREF(dst, src) Py_DECREF(_Py_atomic_exchange_ptr(\u0026(dst), (src)))\n#define Py_XSETREF(dst, src) Py_XDECREF(_Py_atomic_exchange_ptr(\u0026(dst), (src)))\n```\n\nPlease poke holes in this approach. I have a lot of experience with writing multithreaded C code, but will never be convinced I've covered every race condition no matter how simple the code.\n\n### Has this already been discussed elsewhere?\n\nNo response given\n\n### Links to previous discussion of this feature:\n\n_No response_","author":{"url":"https://github.com/MojoVampire","@type":"Person","name":"MojoVampire"},"datePublished":"2026-05-19T00:36:18.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/150044/cpython/issues/150044"}