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merged 5 commits into from
Jan 29, 2025
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gh-129236: Use stackpointer in free threaded GC #129240

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1ec055b
gh-129236: Use `stackpointer` in free threaded GC
colesbury Jan 23, 2025
fcaee80
Remove additional frame initialization
colesbury Jan 24, 2025
ca32c7b
Update Python/gc_free_threading.c
colesbury Jan 24, 2025
1c8a2e3
Update comment
colesbury Jan 24, 2025
57a34c2
Update comment and logic in mark_heap_visitor
colesbury Jan 24, 2025
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24 changes: 0 additions & 24 deletions Include/internal/pycore_frame.h
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Original file line number Diff line number Diff line change
Expand Up @@ -159,13 +159,6 @@ static inline void _PyFrame_Copy(_PyInterpreterFrame *src, _PyInterpreterFrame *
// Don't leave a dangling pointer to the old frame when creating generators
// and coroutines:
dest->previous = NULL;

#ifdef Py_GIL_DISABLED
PyCodeObject *co = _PyFrame_GetCode(dest);
for (int i = stacktop; i < co->co_nlocalsplus + co->co_stacksize; i++) {
dest->localsplus[i] = PyStackRef_NULL;
}
#endif
}

#ifdef Py_GIL_DISABLED
Expand Down Expand Up @@ -219,16 +212,6 @@ _PyFrame_Initialize(
for (int i = null_locals_from; i < code->co_nlocalsplus; i++) {
frame->localsplus[i] = PyStackRef_NULL;
}

#ifdef Py_GIL_DISABLED
// On GIL disabled, we walk the entire stack in GC. Since stacktop
// is not always in sync with the real stack pointer, we have
// no choice but to traverse the entire stack.
// This just makes sure we don't pass the GC invalid stack values.
for (int i = code->co_nlocalsplus; i < code->co_nlocalsplus + code->co_stacksize; i++) {
frame->localsplus[i] = PyStackRef_NULL;
}
#endif
}

/* Gets the pointer to the locals array
Expand Down Expand Up @@ -399,13 +382,6 @@ _PyFrame_PushTrampolineUnchecked(PyThreadState *tstate, PyCodeObject *code, int
frame->owner = FRAME_OWNED_BY_THREAD;
frame->visited = 0;
frame->return_offset = 0;

#ifdef Py_GIL_DISABLED
assert(code->co_nlocalsplus == 0);
for (int i = 0; i < code->co_stacksize; i++) {
frame->localsplus[i] = PyStackRef_NULL;
}
#endif
return frame;
}

Expand Down
78 changes: 50 additions & 28 deletions Python/gc_free_threading.c
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Original file line number Diff line number Diff line change
Expand Up @@ -67,6 +67,10 @@ struct collection_state {
PyInterpreterState *interp;
GCState *gcstate;
_PyGC_Reason reason;
// GH-129236: If we see a an active frame without a valid stack pointer,
// we can't collect objects with deferred references because we may not
// see all references.
int skip_deferred_objects;
Py_ssize_t collected;
Py_ssize_t uncollectable;
Py_ssize_t long_lived_total;
Expand Down Expand Up @@ -413,9 +417,6 @@ gc_visit_heaps(PyInterpreterState *interp, mi_block_visit_fun *visitor,
static inline void
gc_visit_stackref(_PyStackRef stackref)
{
// Note: we MUST check that it is deferred before checking the rest.
// Otherwise we might read into invalid memory due to non-deferred references
// being dead already.
if (PyStackRef_IsDeferred(stackref) && !PyStackRef_IsNull(stackref)) {
PyObject *obj = PyStackRef_AsPyObjectBorrow(stackref);
if (_PyObject_GC_IS_TRACKED(obj) && !gc_is_frozen(obj)) {
Expand All @@ -426,20 +427,26 @@ gc_visit_stackref(_PyStackRef stackref)

// Add 1 to the gc_refs for every deferred reference on each thread's stack.
static void
gc_visit_thread_stacks(PyInterpreterState *interp)
gc_visit_thread_stacks(PyInterpreterState *interp, struct collection_state *state)
{
_Py_FOR_EACH_TSTATE_BEGIN(interp, p) {
for (_PyInterpreterFrame *f = p->current_frame; f != NULL; f = f->previous) {
PyObject *executable = PyStackRef_AsPyObjectBorrow(f->f_executable);
if (executable == NULL || !PyCode_Check(executable)) {
if (f->owner >= FRAME_OWNED_BY_INTERPRETER) {
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I also updated this to match the condition in Python/gc.c.

continue;
}

_PyStackRef *top = f->stackpointer;
if (top == NULL) {
// GH-129236: The stackpointer may be NULL. Skip this frame
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Do we know the cases when stackpointer can be NULL here?

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I see it can be because of PyStackRef_CLOSE, perhaps add that here too or hint to check gc_visit_thread_stacks_mark_alive?

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I've updated the comment.

// and don't collect objects with deferred references.
state->skip_deferred_objects = 1;
continue;
}

PyCodeObject *co = (PyCodeObject *)executable;
int max_stack = co->co_nlocalsplus + co->co_stacksize;
gc_visit_stackref(f->f_executable);
for (int i = 0; i < max_stack; i++) {
gc_visit_stackref(f->localsplus[i]);
while (top != f->localsplus) {
--top;
gc_visit_stackref(*top);
}
}
}
Expand Down Expand Up @@ -519,10 +526,7 @@ gc_abort_mark_alive(PyInterpreterState *interp,
static int
gc_visit_stackref_mark_alive(_PyObjectStack *stack, _PyStackRef stackref)
{
// Note: we MUST check that it is deferred before checking the rest.
// Otherwise we might read into invalid memory due to non-deferred references
// being dead already.
if (PyStackRef_IsDeferred(stackref) && !PyStackRef_IsNull(stackref)) {
if (!PyStackRef_IsNull(stackref)) {
PyObject *op = PyStackRef_AsPyObjectBorrow(stackref);
if (mark_alive_stack_push(op, stack) < 0) {
return -1;
Expand All @@ -534,27 +538,37 @@ gc_visit_stackref_mark_alive(_PyObjectStack *stack, _PyStackRef stackref)
static int
gc_visit_thread_stacks_mark_alive(PyInterpreterState *interp, _PyObjectStack *stack)
{
int err = 0;
_Py_FOR_EACH_TSTATE_BEGIN(interp, p) {
for (_PyInterpreterFrame *f = p->current_frame; f != NULL; f = f->previous) {
PyObject *executable = PyStackRef_AsPyObjectBorrow(f->f_executable);
if (executable == NULL || !PyCode_Check(executable)) {
if (f->owner >= FRAME_OWNED_BY_INTERPRETER) {
continue;
}

PyCodeObject *co = (PyCodeObject *)executable;
int max_stack = co->co_nlocalsplus + co->co_stacksize;
if (f->stackpointer == NULL) {
// GH-129236: The stackpointer may be NULL in cases where
// the GC is run during a PyStackRef_CLOSE() call. Skip this
// frame for now.
continue;
}

_PyStackRef *top = f->stackpointer;
if (gc_visit_stackref_mark_alive(stack, f->f_executable) < 0) {
return -1;
err = -1;
goto exit;
}
for (int i = 0; i < max_stack; i++) {
if (gc_visit_stackref_mark_alive(stack, f->localsplus[i]) < 0) {
return -1;
while (top != f->localsplus) {
--top;
if (gc_visit_stackref_mark_alive(stack, *top) < 0) {
err = -1;
goto exit;
}
}
}
}
exit:
_Py_FOR_EACH_TSTATE_END(interp);
return 0;
return err;
}
#endif // GC_MARK_ALIVE_STACKS
#endif // GC_ENABLE_MARK_ALIVE
Expand Down Expand Up @@ -789,14 +803,22 @@ mark_heap_visitor(const mi_heap_t *heap, const mi_heap_area_t *area,
return true;
}

if (gc_is_alive(op)) {
_PyObject_ASSERT_WITH_MSG(op, gc_get_refs(op) >= 0,
"refcount is too small");

if (gc_is_alive(op) || !gc_is_unreachable(op)) {
// Object was already marked as reachable.
return true;
}

_PyObject_ASSERT_WITH_MSG(op, gc_get_refs(op) >= 0,
"refcount is too small");
// GH-129236: Hack to avoid collect objects with deferred references
// if we see a frame without a valid stack pointer.
struct collection_state *state = (struct collection_state *)args;
if (state->skip_deferred_objects && _PyObject_HasDeferredRefcount(op)) {
gc_add_refs(op, 1);
}

if (gc_is_unreachable(op) && gc_get_refs(op) != 0) {
if (gc_get_refs(op) != 0) {
// Object is reachable but currently marked as unreachable.
// Mark it as reachable and traverse its pointers to find
// any other object that may be directly reachable from it.
Expand Down Expand Up @@ -985,7 +1007,7 @@ deduce_unreachable_heap(PyInterpreterState *interp,
#endif

// Visit the thread stacks to account for any deferred references.
gc_visit_thread_stacks(interp);
gc_visit_thread_stacks(interp, state);

// Transitively mark reachable objects by clearing the
// _PyGC_BITS_UNREACHABLE flag.
Expand Down
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