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gh-159564: Make weakref dereference lock-free. by nascheme · Pull Request #159578 · python/cpython · GitHub

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gh-159564: Make weakref dereference lock-free. #159578

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gh-159564: Make weakref dereference lock-free. #159578

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21 changes: 11 additions & 10 deletions Include/internal/pycore_weakref.h
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Original file line number Diff line number Diff line change
Expand Up @@ -74,26 +74,27 @@ static inline PyObject* _PyWeakref_GET_REF(PyObject *ref_obj)
assert(PyWeakref_Check(ref_obj));
PyWeakReference *ref = _Py_CAST(PyWeakReference*, ref_obj);

PyObject *obj = FT_ATOMIC_LOAD_PTR(ref->wr_object);
#ifdef Py_GIL_DISABLED
PyObject *obj = _Py_atomic_load_ptr(&ref->wr_object);
if (obj == Py_None) {
// clear_weakref() was called
return NULL;
}

LOCK_WEAKREFS(obj);
#ifdef Py_GIL_DISABLED
if (ref->wr_object == Py_None) {
if (_Py_TryIncrefCompare(&ref->wr_object, obj)) {
return obj;
}
return NULL;
#else
PyObject *obj = ref->wr_object;
if (obj == Py_None) {
// clear_weakref() was called
UNLOCK_WEAKREFS(obj);
return NULL;
}
#endif
if (_Py_TryIncref(obj)) {

kumaraditya303 Oct 20, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

This can be a simple incref now
UNLOCK_WEAKREFS(obj);
return obj;
}
UNLOCK_WEAKREFS(obj);
return NULL;
#endif
}

static inline int _PyWeakref_IS_DEAD(PyObject *ref_obj)
Expand All @@ -108,8 +109,8 @@ static inline int _PyWeakref_IS_DEAD(PyObject *ref_obj)
}
else {
LOCK_WEAKREFS(obj);
// See _PyWeakref_GET_REF() for the rationale of this test
#ifdef Py_GIL_DISABLED
// The weakref may have been cleared before the lock was acquired.
ret = (ref->wr_object == Py_None) || _is_dead(obj);
#else
ret = _is_dead(obj);
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-10-16-18-25-48.gh-issue-159564.WMRRFA.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Improve the performance and thread scaling of dereferencing weak references
in the :term:`free-threaded build`. Dereferencing a weak reference no longer
takes a lock.
4 changes: 2 additions & 2 deletions Objects/weakrefobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -84,9 +84,9 @@ clear_weakref_lock_held(PyWeakReference *self, PyObject **callback)
/* If 'self' is the end of the list (and thus self->wr_next ==
NULL) then the weakref list itself (and thus the value of *list)
will end up being set to NULL. */
FT_ATOMIC_STORE_PTR(*list, self->wr_next);
FT_ATOMIC_STORE_PTR_RELEASE(*list, self->wr_next);
}
FT_ATOMIC_STORE_PTR(self->wr_object, Py_None);
FT_ATOMIC_STORE_PTR_RELEASE(self->wr_object, Py_None);
if (self->wr_prev != NULL) {
self->wr_prev->wr_next = self->wr_next;
}
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