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gh-153881: Atomically load dk_nentries in _PyObject_IsInstanceDictEmpty #153882

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gh-153881: Atomically load dk_nentries in _PyObject_IsInstanceDictEmpty #153882

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20 changes: 20 additions & 0 deletions Lib/test/test_free_threading/test_dict.py
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Original file line number Diff line number Diff line change
Expand Up @@ -336,5 +336,25 @@ def reader():
with threading_helper.start_threads([t1, t2]):
pass

def test_getstate_race_with_shared_keys(self):
box = [None]
enter = Barrier(2)
leave = Barrier(2)

def reader():
for _ in range(1000):
enter.wait()
box[0].__getstate__()
leave.wait()

def writer():
for i in range(1000):
box[0] = type(f"C{i}", (), {})()
enter.wait()
setattr(box[0], str(i), 1)
leave.wait()

threading_helper.run_concurrently([reader, writer])

if __name__ == "__main__":
unittest.main()
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-07-17-22-03-39.gh-issue-153881.oDa06s.rst
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@@ -0,0 +1 @@
Fix potential data race when calling :meth:`object.__getstate__`.

@ByteFlowing1337 ByteFlowing1337 Jul 18, 2026

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Suggested change
Fix potential data race when calling :meth:`object.__getstate__`.
Fix potential data race when calling :meth:`object.__getstate__` under the free-threaded build.

I think we should make it clear to users that this data race is only possible under the FT build.

@ZeroIntensity ZeroIntensity Jul 18, 2026

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Use term:`free-threaded build` as well.

2 changes: 1 addition & 1 deletion Objects/dictobject.c
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Expand Up @@ -7722,7 +7722,7 @@ _PyObject_IsInstanceDictEmpty(PyObject *obj)
PyDictValues *values = _PyObject_InlineValues(obj);
if (FT_ATOMIC_LOAD_UINT8(values->valid)) {
PyDictKeysObject *keys = CACHED_KEYS(tp);
for (Py_ssize_t i = 0; i < keys->dk_nentries; i++) {
for (Py_ssize_t i = 0; i < LOAD_KEYS_NENTRIES(keys); i++) {
if (FT_ATOMIC_LOAD_PTR_RELAXED(values->values[i]) != NULL) {
return 0;
}
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