gh-128100: Atomic dict load in _PyObject_GenericGetAttrWithDict #128297
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Objects/object.c
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| @@ -1715,10 +1715,16 @@ _PyObject_GenericGetAttrWithDict(PyObject *obj, PyObject *name, | |||
| dict = (PyObject *)_PyObject_GetManagedDict(obj); | |||
| } | |||
| else { | |||
| Py_BEGIN_CRITICAL_SECTION(obj); | |||
| #ifdef DISABLE_GIL | |||
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This should be Py_GIL_DISABLED.
Objects/object.c
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| @@ -1715,10 +1715,16 @@ _PyObject_GenericGetAttrWithDict(PyObject *obj, PyObject *name, | |||
| dict = (PyObject *)_PyObject_GetManagedDict(obj); | |||
| } | |||
| else { | |||
| Py_BEGIN_CRITICAL_SECTION(obj); | |||
| #ifdef DISABLE_GIL | |||
| 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.
Objects/object.c
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| @@ -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.
| 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.)
colesbury
approved these changes
Dec 30, 2024
It wasn't reporting any TSAN failures before the PR.
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This is my very first attempt in free-threading in cpython. So that I would be happy to get some criticism