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[3.13] gh-127582: Make object resurrection thread-safe for free threading. (GH-127612) (GH-127659)

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Objects may be temporarily "resurrected" in destructors when calling
finalizers or watcher callbacks. We previously undid the resurrection
by decrementing the reference count using `Py_SET_REFCNT`. This was not
thread-safe because other threads might be accessing the object
(modifying its reference count) if it was exposed by the finalizer,
watcher callback, or temporarily accessed by a racy dictionary or list
access.

This adds internal-only thread-safe functions for temporary object
resurrection during destructors.
(cherry picked from commit f4f530804b)
This commit is contained in:
Sam Gross 2024-12-05 23:49:33 +00:00 • committed by GitHub
parent 304111e967
commit 69bb1c6c41
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GPG key ID: B5690EEEBB952194
6 changed files with 87 additions and 20 deletions
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44
Include/internal/pycore_object.h
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@ -572,8 +572,52 @@ _PyObject_SetMaybeWeakref(PyObject *op)
}
}
extern int _PyObject_ResurrectEndSlow(PyObject *op);
#endif
// Temporarily resurrects an object during deallocation. The refcount is set
// to one.
static inline void
_PyObject_ResurrectStart(PyObject *op)
{
assert(Py_REFCNT(op) == 0);
#ifdef Py_REF_DEBUG
_Py_IncRefTotal(_PyThreadState_GET());
#endif
#ifdef Py_GIL_DISABLED
_Py_atomic_store_uintptr_relaxed(&op->ob_tid, _Py_ThreadId());
_Py_atomic_store_uint32_relaxed(&op->ob_ref_local, 1);
_Py_atomic_store_ssize_relaxed(&op->ob_ref_shared, 0);
#else
Py_SET_REFCNT(op, 1);
#endif
}
// Undoes an object resurrection by decrementing the refcount without calling
// _Py_Dealloc(). Returns 0 if the object is dead (the normal case), and
// deallocation should continue. Returns 1 if the object is still alive.
static inline int
_PyObject_ResurrectEnd(PyObject *op)
{
#ifdef Py_REF_DEBUG
_Py_DecRefTotal(_PyThreadState_GET());
#endif
#ifndef Py_GIL_DISABLED
Py_SET_REFCNT(op, Py_REFCNT(op) - 1);
return Py_REFCNT(op) != 0;
#else
uint32_t local = _Py_atomic_load_uint32_relaxed(&op->ob_ref_local);
Py_ssize_t shared = _Py_atomic_load_ssize_acquire(&op->ob_ref_shared);
if (_Py_IsOwnedByCurrentThread(op) && local == 1 && shared == 0) {
// Fast-path: object has a single refcount and is owned by this thread
_Py_atomic_store_uint32_relaxed(&op->ob_ref_local, 0);
return 0;
}
// Slow-path: object has a shared refcount or is not owned by this thread
return _PyObject_ResurrectEndSlow(op);
#endif
}
/* Tries to incref op and returns 1 if successful or 0 otherwise. */
static inline int
_Py_TryIncref(PyObject *op)