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Commit 3094372

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naschemenascheme
committed
gh-127266: avoid data races when updating type slots
In the free-threaded build, avoid data races caused by updating type slots or type flags after the type was initially created. For those (typically rare) cases, use the stop-the-world mechanism. Remove the use of atomics when reading or writing type flags. The use of atomics is not sufficient to avoid races (since flags are sometimes read without a lock and without atomics) and are no longer required.
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‎Include/internal/pycore_interp_structs.h

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@@ -667,8 +667,11 @@ struct _Py_interp_cached_objects {
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/* object.__reduce__ */
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PyObject *objreduce;
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#ifndef Py_GIL_DISABLED
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/* resolve_slotdups() */
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PyObject *type_slots_pname;
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pytype_slotdef *type_slots_ptrs[MAX_EQUIV];
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#endif
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/* TypeVar and related types */
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PyTypeObject *generic_type;

‎Include/internal/pycore_object.h

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+1-1Lines changed: 1 addition & 1 deletion
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@@ -313,7 +313,7 @@ extern int _PyDict_CheckConsistency(PyObject *mp, int check_content);
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// Fast inlined version of PyType_HasFeature()
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static inline int
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_PyType_HasFeature(PyTypeObject *type, unsigned long feature) {
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return ((FT_ATOMIC_LOAD_ULONG_RELAXED(type->tp_flags) & feature) != 0);
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return ((type->tp_flags) & feature) != 0;
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}
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extern void _PyType_InitCache(PyInterpreterState *interp);

‎Include/internal/pycore_typeobject.h

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@@ -134,7 +134,6 @@ extern int _PyType_AddMethod(PyTypeObject *, PyMethodDef *);
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extern void _PyType_SetFlagsRecursive(PyTypeObject *self, unsigned long mask,
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unsigned long flags);
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extern unsigned int _PyType_GetVersionForCurrentState(PyTypeObject *tp);
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PyAPI_FUNC(void) _PyType_SetVersion(PyTypeObject *tp, unsigned int version);
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PyTypeObject *_PyType_LookupByVersion(unsigned int version);
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‎Include/object.h

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+7-5Lines changed: 7 additions & 5 deletions
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@@ -620,6 +620,12 @@ given type object has a specified feature.
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#define Py_TPFLAGS_HAVE_FINALIZE (1UL << 0)
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#define Py_TPFLAGS_HAVE_VERSION_TAG (1UL << 18)
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// Flag values for ob_flags (16 bits available, if SIZEOF_VOID_P > 4).
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#define _Py_IMMORTAL_FLAGS (1 << 0)
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#define _Py_STATICALLY_ALLOCATED_FLAG (1 << 2)
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#if defined(Py_GIL_DISABLED) && defined(Py_DEBUG)
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#define _Py_TYPE_REVEALED_FLAG (1 << 3)
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#endif
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#define Py_CONSTANT_NONE 0
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#define Py_CONSTANT_FALSE 1
@@ -776,11 +782,7 @@ PyType_HasFeature(PyTypeObject *type, unsigned long feature)
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// PyTypeObject is opaque in the limited C API
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flags = PyType_GetFlags(type);
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#else
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# ifdef Py_GIL_DISABLED
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flags = _Py_atomic_load_ulong_relaxed(&type->tp_flags);
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# else
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flags = type->tp_flags;
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# endif
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flags = type->tp_flags;
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#endif
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return ((flags & feature) != 0);
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}

‎Include/refcount.h

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@@ -19,9 +19,6 @@ immortal. The latter should be the only instances that require
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cleanup during runtime finalization.
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*/
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#define _Py_STATICALLY_ALLOCATED_FLAG 4
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#define _Py_IMMORTAL_FLAGS 1
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#if SIZEOF_VOID_P > 4
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/*
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In 64+ bit systems, any object whose 32 bit reference count is >= 2**31

‎Misc/NEWS.d/next/Core_and_Builtins/2025-03-14-13-08-20.gh-issue-127266._tyfBp.rst

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@@ -0,0 +1,6 @@
1+
In the free-threaded build, avoid data races caused by updating type slots
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or type flags after the type was initially created. For those (typically
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rare) cases, use the stop-the-world mechanism. Remove the use of atomics
4+
when reading or writing type flags. The use of atomics is not sufficient to
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avoid races (since flags are sometimes read without a lock and without
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atomics) and are no longer required.

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