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

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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.
This commit is contained in:
Sam Gross 2024-12-05 21:07:31 +00:00 • committed by GitHub
parent 657d0e99aa
commit f4f530804b
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GPG key ID: B5690EEEBB952194
6 changed files with 87 additions and 20 deletions
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40
Objects/object.c
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@ -362,8 +362,10 @@ is_dead(PyObject *o)
}
# endif
void
_Py_DecRefSharedDebug(PyObject *o, const char *filename, int lineno)
// Decrement the shared reference count of an object. Return 1 if the object
// is dead and should be deallocated, 0 otherwise.
static int
_Py_DecRefSharedIsDead(PyObject *o, const char *filename, int lineno)
{
// Should we queue the object for the owning thread to merge?
int should_queue;
@ -404,6 +406,15 @@ _Py_DecRefSharedDebug(PyObject *o, const char *filename, int lineno)
}
else if (new_shared == _Py_REF_MERGED) {
// refcount is zero AND merged
return 1;
}
return 0;
}
void
_Py_DecRefSharedDebug(PyObject *o, const char *filename, int lineno)
{
if (_Py_DecRefSharedIsDead(o, filename, lineno)) {
_Py_Dealloc(o);
}
}
@ -472,6 +483,26 @@ _Py_ExplicitMergeRefcount(PyObject *op, Py_ssize_t extra)
&shared, new_shared));
return refcnt;
}
// The more complicated "slow" path for undoing the resurrection of an object.
int
_PyObject_ResurrectEndSlow(PyObject *op)
{
if (_Py_IsImmortal(op)) {
return 1;
}
if (_Py_IsOwnedByCurrentThread(op)) {
// If the object is owned by the current thread, give up ownership and
// merge the refcount. This isn't necessary in all cases, but it
// simplifies the implementation.
Py_ssize_t refcount = _Py_ExplicitMergeRefcount(op, -1);
return refcount != 0;
}
int is_dead = _Py_DecRefSharedIsDead(op, NULL, 0);
return !is_dead;
}
#endif /* Py_GIL_DISABLED */
@ -550,7 +581,7 @@ PyObject_CallFinalizerFromDealloc(PyObject *self)
}
/* Temporarily resurrect the object. */
Py_SET_REFCNT(self, 1);
_PyObject_ResurrectStart(self);
PyObject_CallFinalizer(self);
@ -560,8 +591,7 @@ PyObject_CallFinalizerFromDealloc(PyObject *self)
/* Undo the temporary resurrection; can't use DECREF here, it would
* cause a recursive call. */
Py_SET_REFCNT(self, Py_REFCNT(self) - 1);
if (Py_REFCNT(self) == 0) {
if (!_PyObject_ResurrectEnd(self)) {
return 0; /* this is the normal path out */
}