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Title: gh-151646: Fix data race between gc.get_stats() and GC in free-threading builds by zangjiucheng · Pull Request #151766 · python/cpython · GitHub

Open Graph Title: gh-151646: Fix data race between gc.get_stats() and GC in free-threading builds by zangjiucheng · Pull Request #151766 · python/cpython

X Title: gh-151646: Fix data race between gc.get_stats() and GC in free-threading builds by zangjiucheng · Pull Request #151766 · python/cpython

Description: In free-threading builds, gc_get_stats_impl() reads per-generation stats with plain struct copies while gc_collect_main() writes the same fields after _Py_StartTheWorld() — no synchronisation on either side. Fix: add PyMutex stats_mutex to _gc_runtime_state (under Py_GIL_DISABLED) and hold it around both the seven field writes in the writer and the three struct copies in the reader. The lock is acquired after _Py_StartTheWorld(), so no deadlock with stop-the-world. GIL build is unchanged. Also adds Lib/test/test_free_threading/test_gc_get_stats_race.py. Verified with TSAN on Linux: race confirmed on main, clean on the patch. 🤖 Generated with Claude Code Issue: gh-151646

Open Graph Description: In free-threading builds, gc_get_stats_impl() reads per-generation stats with plain struct copies while gc_collect_main() writes the same fields after _Py_StartTheWorld() — no synchronisation on ei...

X Description: In free-threading builds, gc_get_stats_impl() reads per-generation stats with plain struct copies while gc_collect_main() writes the same fields after _Py_StartTheWorld() — no synchronisation on ei...

Opengraph URL: https://github.com/python/cpython/pull/151766

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47cfd29 Fix data race in free-threading builds between gc.get_stats() and gar… zangjiucheng Jun 20, 2026 https://github.com/python/cpython/pull/151766/commits/47cfd29848271a8d379959d879d9e03b7b3a1765
08a114a gh-151646: Lighten gc.get_stats race test and trim mutex comments zangjiucheng Jul 4, 2026 https://github.com/python/cpython/pull/151766/commits/08a114aef1d12927ec80f4d57511bda6b0f84ad5
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