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gh-125859: Fix crash when gc.get_objects is called during GC (#125882)

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This fixes a crash when `gc.get_objects()` or `gc.get_referrers()` is
called during a GC in the free threading build.

Switch to `_PyObjectStack` to avoid corrupting the `struct worklist`
linked list maintained by the GC. Also, don't return objects that are frozen
(`gc.freeze()`) or in the process of being collected to more closely match
the behavior of the default build.
This commit is contained in:
Sam Gross 2024-10-24 09:33:11 -04:00 • committed by GitHub
parent b61fece852
commit e545ead66c
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GPG key ID: B5690EEEBB952194
5 changed files with 160 additions and 73 deletions
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137
Python/gc_free_threading.c
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@ -1401,10 +1401,32 @@ gc_collect_main(PyThreadState *tstate, int generation, _PyGC_Reason reason)
return n + m;
}
static PyObject *
list_from_object_stack(_PyObjectStack *stack)
{
PyObject *list = PyList_New(_PyObjectStack_Size(stack));
if (list == NULL) {
PyObject *op;
while ((op = _PyObjectStack_Pop(stack)) != NULL) {
Py_DECREF(op);
}
return NULL;
}
PyObject *op;
Py_ssize_t idx = 0;
while ((op = _PyObjectStack_Pop(stack)) != NULL) {
assert(idx < PyList_GET_SIZE(list));
PyList_SET_ITEM(list, idx++, op);
}
assert(idx == PyList_GET_SIZE(list));
return list;
}
struct get_referrers_args {
struct visitor_args base;
PyObject *objs;
struct worklist results;
_PyObjectStack results;
};
static int
@ -1428,11 +1450,21 @@ visit_get_referrers(const mi_heap_t *heap, const mi_heap_area_t *area,
if (op == NULL) {
return true;
}
if (op->ob_gc_bits & (_PyGC_BITS_UNREACHABLE | _PyGC_BITS_FROZEN)) {
// Exclude unreachable objects (in-progress GC) and frozen
// objects from gc.get_objects() to match the default build.
return true;
}
struct get_referrers_args *arg = (struct get_referrers_args *)args;
if (op == arg->objs) {
// Don't include the tuple itself in the referrers list.
return true;
}
if (Py_TYPE(op)->tp_traverse(op, referrersvisit, arg->objs)) {
op->ob_tid = 0; // we will restore the refcount later
worklist_push(&arg->results, op);
if (_PyObjectStack_Push(&arg->results, Py_NewRef(op)) < 0) {
return false;
}
}
return true;
@ -1441,48 +1473,25 @@ visit_get_referrers(const mi_heap_t *heap, const mi_heap_area_t *area,
PyObject *
_PyGC_GetReferrers(PyInterpreterState *interp, PyObject *objs)
{
PyObject *result = PyList_New(0);
if (!result) {
return NULL;
}
_PyEval_StopTheWorld(interp);
// Append all objects to a worklist. This abuses ob_tid. We will restore
// it later. NOTE: We can't append to the PyListObject during
// gc_visit_heaps() because PyList_Append() may reclaim an abandoned
// mimalloc segments while we are traversing them.
// NOTE: We can't append to the PyListObject during gc_visit_heaps()
// because PyList_Append() may reclaim an abandoned mimalloc segments
// while we are traversing them.
struct get_referrers_args args = { .objs = objs };
gc_visit_heaps(interp, &visit_get_referrers, &args.base);
bool error = false;
PyObject *op;
while ((op = worklist_pop(&args.results)) != NULL) {
gc_restore_tid(op);
if (op != objs && PyList_Append(result, op) < 0) {
error = true;
break;
}
}
// In case of error, clear the remaining worklist
while ((op = worklist_pop(&args.results)) != NULL) {
gc_restore_tid(op);
}
_PyEval_StopTheWorld(interp);
int err = gc_visit_heaps(interp, &visit_get_referrers, &args.base);
_PyEval_StartTheWorld(interp);
if (error) {
Py_DECREF(result);
return NULL;
PyObject *list = list_from_object_stack(&args.results);
if (err < 0) {
PyErr_NoMemory();
Py_CLEAR(list);
}
return result;
return list;
}
struct get_objects_args {
struct visitor_args base;
struct worklist objects;
_PyObjectStack objects;
};
static bool
@ -1493,54 +1502,36 @@ visit_get_objects(const mi_heap_t *heap, const mi_heap_area_t *area,
if (op == NULL) {
return true;
}
if (op->ob_gc_bits & (_PyGC_BITS_UNREACHABLE | _PyGC_BITS_FROZEN)) {
// Exclude unreachable objects (in-progress GC) and frozen
// objects from gc.get_objects() to match the default build.
return true;
}
struct get_objects_args *arg = (struct get_objects_args *)args;
op->ob_tid = 0; // we will restore the refcount later
worklist_push(&arg->objects, op);
if (_PyObjectStack_Push(&arg->objects, Py_NewRef(op)) < 0) {
return false;
}
return true;
}
PyObject *
_PyGC_GetObjects(PyInterpreterState *interp, int generation)
{
PyObject *result = PyList_New(0);
if (!result) {
return NULL;
}
_PyEval_StopTheWorld(interp);
// Append all objects to a worklist. This abuses ob_tid. We will restore
// it later. NOTE: We can't append to the list during gc_visit_heaps()
// because PyList_Append() may reclaim an abandoned mimalloc segment
// while we are traversing it.
// NOTE: We can't append to the PyListObject during gc_visit_heaps()
// because PyList_Append() may reclaim an abandoned mimalloc segments
// while we are traversing them.
struct get_objects_args args = { 0 };
gc_visit_heaps(interp, &visit_get_objects, &args.base);
bool error = false;
PyObject *op;
while ((op = worklist_pop(&args.objects)) != NULL) {
gc_restore_tid(op);
if (op != result && PyList_Append(result, op) < 0) {
error = true;
break;
}
}
// In case of error, clear the remaining worklist
while ((op = worklist_pop(&args.objects)) != NULL) {
gc_restore_tid(op);
}
_PyEval_StopTheWorld(interp);
int err = gc_visit_heaps(interp, &visit_get_objects, &args.base);
_PyEval_StartTheWorld(interp);
if (error) {
Py_DECREF(result);
return NULL;
PyObject *list = list_from_object_stack(&args.objects);
if (err < 0) {
PyErr_NoMemory();
Py_CLEAR(list);
}
return result;
return list;
}
static bool