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gh-128100: Atomic dict load in _PyObject_GenericGetAttrWithDict #128297

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Merged
merged 4 commits into from
Dec 30, 2024
Merged

gh-128100: Atomic dict load in _PyObject_GenericGetAttrWithDict #128297

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c8ccd02
Implement atomic load and critical section in C code
wrongnull Dec 26, 2024
3d0d584
Add tests
wrongnull Dec 26, 2024
7227e3e
Address the review
wrongnull Dec 27, 2024
b92759a
Remove test case.
colesbury Dec 30, 2024
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19 changes: 19 additions & 0 deletions Lib/test/test_free_threading/test_races.py
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Original file line number Diff line number Diff line change
Expand Up @@ -270,6 +270,25 @@ def mutate():

do_race(set_value, mutate)

def test_generic_getattr(self):
"""Test generic attribute load"""
import concurrent.futures

num_threads = 100


def closure(b, o):
b.wait()
getattr(o, "foo", None)
o.foo = 42

with concurrent.futures.ThreadPoolExecutor(max_workers=num_threads) as executor:
for _ in range(100):
b = threading.Barrier(num_threads)
o = functools.partial(lambda x: x, 42)
for _ in range(num_threads):
executor.submit(functools.partial(closure, b, o))


if __name__ == "__main__":
unittest.main()
6 changes: 6 additions & 0 deletions Objects/object.c
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Expand Up @@ -1715,10 +1715,16 @@ _PyObject_GenericGetAttrWithDict(PyObject *obj, PyObject *name,
dict = (PyObject *)_PyObject_GetManagedDict(obj);
}
else {
Py_BEGIN_CRITICAL_SECTION(obj);
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In general, we don't need to lock something if we're going to use one of the _Py_atomic APIs, and vice versa.

#ifdef DISABLE_GIL
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This should be Py_GIL_DISABLED.

PyObject **dictptr = _Py_atomic_load_ptr(_PyObject_ComputedDictPointer(obj));
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This is the wrong thing to load atomically--dereferencing it should be the atomic operation. In fact, this doesn't do what you think it does: _Py_atomic_load* takes a pointer, and the thing that it points to gets atomically loaded and returned.

#else
PyObject **dictptr = _PyObject_ComputedDictPointer(obj);
#endif
if (dictptr) {
dict = *dictptr;
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Here is where the atomic load should be. (And, as Sam said in the issue, we want _Py_atomic_load_ptr_acquire instead of plain old _Py_atomic_load_ptr.)

}
Py_END_CRITICAL_SECTION();
}
}
if (dict != NULL) {
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