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[3.13] gh-119999: Fix potential race condition in _Py_ExplicitMergeRefcount (GH-120000) #120073

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Merged
colesbury merged 1 commit into from
Jun 4, 2024
Merged

[3.13] gh-119999: Fix potential race condition in _Py_ExplicitMergeRefcount (GH-120000) #120073

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19 changes: 11 additions & 8 deletions Objects/object.c
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Original file line number Diff line number Diff line change
Expand Up @@ -401,24 +401,27 @@ Py_ssize_t
_Py_ExplicitMergeRefcount(PyObject *op, Py_ssize_t extra)
{
assert(!_Py_IsImmortal(op));

#ifdef Py_REF_DEBUG
_Py_AddRefTotal(_PyThreadState_GET(), extra);
#endif

// gh-119999: Write to ob_ref_local and ob_tid before merging the refcount.
Py_ssize_t local = (Py_ssize_t)op->ob_ref_local;
_Py_atomic_store_uint32_relaxed(&op->ob_ref_local, 0);
_Py_atomic_store_uintptr_relaxed(&op->ob_tid, 0);

Py_ssize_t refcnt;
Py_ssize_t new_shared;
Py_ssize_t shared = _Py_atomic_load_ssize_relaxed(&op->ob_ref_shared);
do {
refcnt = Py_ARITHMETIC_RIGHT_SHIFT(Py_ssize_t, shared, _Py_REF_SHARED_SHIFT);
refcnt += (Py_ssize_t)op->ob_ref_local;
refcnt += local;
refcnt += extra;

new_shared = _Py_REF_SHARED(refcnt, _Py_REF_MERGED);
} while (!_Py_atomic_compare_exchange_ssize(&op->ob_ref_shared,
&shared, new_shared));

#ifdef Py_REF_DEBUG
_Py_AddRefTotal(_PyThreadState_GET(), extra);
#endif

_Py_atomic_store_uint32_relaxed(&op->ob_ref_local, 0);
_Py_atomic_store_uintptr_relaxed(&op->ob_tid, 0);
return refcnt;
}
#endif /* Py_GIL_DISABLED */
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