gh-123923: Defer refcounting for f_executable in _PyInterpreterFrame (#123924)

Use a `_PyStackRef` and defer the reference to `f_executable` when
possible. This avoids some reference count contention in the common case
of executing the same code object from multiple threads concurrently in
the free-threaded build.

Python/bytecodes.c

@@ -3598,7 +3598,7 @@ dummy_func(
         op(_CREATE_INIT_FRAME, (self, init, args[oparg] -- init_frame: _PyInterpreterFrame *)) {
             _PyInterpreterFrame *shim = _PyFrame_PushTrampolineUnchecked(
                 tstate, (PyCodeObject *)&_Py_InitCleanup, 1, frame);
-            assert(_PyCode_CODE((PyCodeObject *)shim->f_executable)[0].op.code == EXIT_INIT_CHECK);
+            assert(_PyCode_CODE(_PyFrame_GetCode(shim))[0].op.code == EXIT_INIT_CHECK);
             /* Push self onto stack of shim */
             shim->localsplus[0] = PyStackRef_DUP(self);
             PyFunctionObject *init_func = (PyFunctionObject *)PyStackRef_AsPyObjectSteal(init);
@@ -4798,7 +4798,7 @@ dummy_func(
         #endif
             _PyExecutorObject *executor;
             if (target->op.code == ENTER_EXECUTOR) {
-                PyCodeObject *code = (PyCodeObject *)frame->f_executable;
+                PyCodeObject *code = _PyFrame_GetCode(frame);
                 executor = code->co_executors->executors[target->op.arg];
                 Py_INCREF(executor);
             }

Python/ceval.c

@@ -194,7 +194,7 @@ static void
 lltrace_resume_frame(_PyInterpreterFrame *frame)
 {
     PyObject *fobj = frame->f_funcobj;
-    if (!PyCode_Check(frame->f_executable) ||
+    if (!PyStackRef_CodeCheck(frame->f_executable) ||
         fobj == NULL ||
         !PyFunction_Check(fobj)
     ) {
@@ -784,7 +784,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int
     entry_frame.f_globals = (PyObject*)0xaaa3;
     entry_frame.f_builtins = (PyObject*)0xaaa4;
 #endif
-    entry_frame.f_executable = Py_None;
+    entry_frame.f_executable = PyStackRef_None;
     entry_frame.instr_ptr = (_Py_CODEUNIT *)_Py_INTERPRETER_TRAMPOLINE_INSTRUCTIONS + 1;
     entry_frame.stackpointer = entry_frame.localsplus;
     entry_frame.owner = FRAME_OWNED_BY_CSTACK;
@@ -1681,7 +1681,7 @@ clear_thread_frame(PyThreadState *tstate, _PyInterpreterFrame * frame)
     tstate->c_recursion_remaining--;
     assert(frame->frame_obj == NULL || frame->frame_obj->f_frame == frame);
     _PyFrame_ClearExceptCode(frame);
-    Py_DECREF(frame->f_executable);
+    PyStackRef_CLEAR(frame->f_executable);
     tstate->c_recursion_remaining++;
     _PyThreadState_PopFrame(tstate, frame);
 }

Python/executor_cases.c.h

@@ -4143,7 +4143,7 @@
             self = stack_pointer[-2 - oparg];
             _PyInterpreterFrame *shim = _PyFrame_PushTrampolineUnchecked(
                 tstate, (PyCodeObject *)&_Py_InitCleanup, 1, frame);
-            assert(_PyCode_CODE((PyCodeObject *)shim->f_executable)[0].op.code == EXIT_INIT_CHECK);
+            assert(_PyCode_CODE(_PyFrame_GetCode(shim))[0].op.code == EXIT_INIT_CHECK);
             /* Push self onto stack of shim */
             shim->localsplus[0] = PyStackRef_DUP(self);
             PyFunctionObject *init_func = (PyFunctionObject *)PyStackRef_AsPyObjectSteal(init);
@@ -5413,7 +5413,7 @@
             #endif
             _PyExecutorObject *executor;
             if (target->op.code == ENTER_EXECUTOR) {
-                PyCodeObject *code = (PyCodeObject *)frame->f_executable;
+                PyCodeObject *code = _PyFrame_GetCode(frame);
                 executor = code->co_executors->executors[target->op.arg];
                 Py_INCREF(executor);
             }

Python/frame.c

@@ -14,7 +14,10 @@ _PyFrame_Traverse(_PyInterpreterFrame *frame, visitproc visit, void *arg)
     Py_VISIT(frame->frame_obj);
     Py_VISIT(frame->f_locals);
     Py_VISIT(frame->f_funcobj);
-    Py_VISIT(_PyFrame_GetCode(frame));
+    int err = _PyGC_VisitStackRef(&frame->f_executable, visit, arg);
+    if (err) {
+        return err;
+    }
     return _PyGC_VisitFrameStack(frame, visit, arg);
 }
 
@@ -53,10 +56,10 @@ take_ownership(PyFrameObject *f, _PyInterpreterFrame *frame)
     assert(frame->owner != FRAME_OWNED_BY_FRAME_OBJECT);
     assert(frame->owner != FRAME_CLEARED);
     Py_ssize_t size = ((char*)frame->stackpointer) - (char *)frame;
-    Py_INCREF(_PyFrame_GetCode(frame));
     memcpy((_PyInterpreterFrame *)f->_f_frame_data, frame, size);
     frame = (_PyInterpreterFrame *)f->_f_frame_data;
     frame->stackpointer = (_PyStackRef *)(((char *)frame) + size);
+    frame->f_executable = PyStackRef_DUP(frame->f_executable);
     f->f_frame = frame;
     frame->owner = FRAME_OWNED_BY_FRAME_OBJECT;
     if (_PyFrame_IsIncomplete(frame)) {
@@ -131,9 +134,7 @@ _PyFrame_ClearExceptCode(_PyInterpreterFrame *frame)
 PyObject *
 PyUnstable_InterpreterFrame_GetCode(struct _PyInterpreterFrame *frame)
 {
-    PyObject *code = frame->f_executable;
-    Py_INCREF(code);
-    return code;
+    return PyStackRef_AsPyObjectNew(frame->f_executable);
 }
 
 int

Python/gc.c

@@ -534,6 +534,13 @@ visit_decref(PyObject *op, void *parent)
     return 0;
 }
 
+int
+_PyGC_VisitStackRef(_PyStackRef *ref, visitproc visit, void *arg)
+{
+    Py_VISIT(PyStackRef_AsPyObjectBorrow(*ref));
+    return 0;
+}
+
 int
 _PyGC_VisitFrameStack(_PyInterpreterFrame *frame, visitproc visit, void *arg)
 {

Python/gc_free_threading.c

@@ -161,39 +161,6 @@ gc_decref(PyObject *op)
     op->ob_tid -= 1;
 }
 
-static void
-disable_deferred_refcounting(PyObject *op)
-{
-    if (!_PyObject_HasDeferredRefcount(op)) {
-        return;
-    }
-
-    op->ob_gc_bits &= ~_PyGC_BITS_DEFERRED;
-    op->ob_ref_shared -= _Py_REF_SHARED(_Py_REF_DEFERRED, 0);
-
-    if (PyType_Check(op)) {
-        // Disable thread-local refcounting for heap types
-        PyTypeObject *type = (PyTypeObject *)op;
-        if (PyType_HasFeature(type, Py_TPFLAGS_HEAPTYPE)) {
-            _PyType_ReleaseId((PyHeapTypeObject *)op);
-        }
-    }
-    else if (PyGen_CheckExact(op) || PyCoro_CheckExact(op) || PyAsyncGen_CheckExact(op)) {
-        // Ensure any non-refcounted pointers in locals are converted to
-        // strong references.  This ensures that the generator/coroutine is not
-        // freed before its locals.
-        PyGenObject *gen = (PyGenObject *)op;
-        struct _PyInterpreterFrame *frame = &gen->gi_iframe;
-        assert(frame->stackpointer != NULL);
-        for (_PyStackRef *ref = frame->localsplus; ref < frame->stackpointer; ref++) {
-            if (!PyStackRef_IsNull(*ref) && PyStackRef_IsDeferred(*ref)) {
-                // Convert a deferred reference to a strong reference.
-                *ref = PyStackRef_FromPyObjectSteal(PyStackRef_AsPyObjectSteal(*ref));
-            }
-        }
-    }
-}
-
 static Py_ssize_t
 merge_refcount(PyObject *op, Py_ssize_t extra)
 {
@@ -213,6 +180,51 @@ merge_refcount(PyObject *op, Py_ssize_t extra)
     return refcount;
 }
 
+static void
+frame_disable_deferred_refcounting(_PyInterpreterFrame *frame)
+{
+    // Convert locals, variables, and the executable object to strong
+    // references from (possibly) deferred references.
+    assert(frame->stackpointer != NULL);
+    frame->f_executable = PyStackRef_AsStrongReference(frame->f_executable);
+    for (_PyStackRef *ref = frame->localsplus; ref < frame->stackpointer; ref++) {
+        if (!PyStackRef_IsNull(*ref) && PyStackRef_IsDeferred(*ref)) {
+            *ref = PyStackRef_AsStrongReference(*ref);
+        }
+    }
+}
+
+static void
+disable_deferred_refcounting(PyObject *op)
+{
+    if (_PyObject_HasDeferredRefcount(op)) {
+        op->ob_gc_bits &= ~_PyGC_BITS_DEFERRED;
+        op->ob_ref_shared -= _Py_REF_SHARED(_Py_REF_DEFERRED, 0);
+        merge_refcount(op, 0);
+    }
+
+    // Heap types also use thread-local refcounting -- disable it here.
+    if (PyType_Check(op)) {
+        // Disable thread-local refcounting for heap types
+        PyTypeObject *type = (PyTypeObject *)op;
+        if (PyType_HasFeature(type, Py_TPFLAGS_HEAPTYPE)) {
+            _PyType_ReleaseId((PyHeapTypeObject *)op);
+        }
+    }
+
+    // Generators and frame objects may contain deferred references to other
+    // objects. If the pointed-to objects are part of cyclic trash, we may
+    // have disabled deferred refcounting on them and need to ensure that we
+    // use strong references, in case the generator or frame object is
+    // resurrected by a finalizer.
+    if (PyGen_CheckExact(op) || PyCoro_CheckExact(op) || PyAsyncGen_CheckExact(op)) {
+        frame_disable_deferred_refcounting(&((PyGenObject *)op)->gi_iframe);
+    }
+    else if (PyFrame_Check(op)) {
+        frame_disable_deferred_refcounting(((PyFrameObject *)op)->f_frame);
+    }
+}
+
 static void
 gc_restore_tid(PyObject *op)
 {
@@ -349,16 +361,18 @@ gc_visit_thread_stacks(PyInterpreterState *interp)
 {
     HEAD_LOCK(&_PyRuntime);
     for (PyThreadState *p = interp->threads.head; p != NULL; p = p->next) {
-        _PyInterpreterFrame *f = p->current_frame;
-        while (f != NULL) {
-            if (f->f_executable != NULL && PyCode_Check(f->f_executable)) {
-                PyCodeObject *co = (PyCodeObject *)f->f_executable;
-                int max_stack = co->co_nlocalsplus + co->co_stacksize;
-                for (int i = 0; i < max_stack; i++) {
-                    gc_visit_stackref(f->localsplus[i]);
-                }
+        for (_PyInterpreterFrame *f = p->current_frame; f != NULL; f = f->previous) {
+            PyObject *executable = PyStackRef_AsPyObjectBorrow(f->f_executable);
+            if (executable == NULL || !PyCode_Check(executable)) {
+                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]);
             }
-            f = f->previous;
         }
     }
     HEAD_UNLOCK(&_PyRuntime);
@@ -623,23 +637,19 @@ scan_heap_visitor(const mi_heap_t *heap, const mi_heap_area_t *area,
         else {
             worklist_push(&state->unreachable, op);
         }
+        return true;
     }
-    else if (state->reason == _Py_GC_REASON_SHUTDOWN &&
-             _PyObject_HasDeferredRefcount(op))
-    {
+
+    if (state->reason == _Py_GC_REASON_SHUTDOWN) {
         // Disable deferred refcounting for reachable objects as well during
         // interpreter shutdown. This ensures that these objects are collected
         // immediately when their last reference is removed.
         disable_deferred_refcounting(op);
-        merge_refcount(op, 0);
-        state->long_lived_total++;
-    }
-    else {
-        // object is reachable, restore `ob_tid`; we're done with these objects
-        gc_restore_tid(op);
-        state->long_lived_total++;
     }
 
+    // object is reachable, restore `ob_tid`; we're done with these objects
+    gc_restore_tid(op);
+    state->long_lived_total++;
     return true;
 }
 
@@ -951,20 +961,29 @@ visit_decref_unreachable(PyObject *op, void *data)
     return 0;
 }
 
+int
+_PyGC_VisitStackRef(_PyStackRef *ref, visitproc visit, void *arg)
+{
+    // This is a bit tricky! We want to ignore deferred references when
+    // computing the incoming references, but otherwise treat them like
+    // regular references.
+    if (!PyStackRef_IsDeferred(*ref) ||
+        (visit != visit_decref && visit != visit_decref_unreachable))
+    {
+        Py_VISIT(PyStackRef_AsPyObjectBorrow(*ref));
+    }
+    return 0;
+}
+
 int
 _PyGC_VisitFrameStack(_PyInterpreterFrame *frame, visitproc visit, void *arg)
 {
     _PyStackRef *ref = _PyFrame_GetLocalsArray(frame);
     /* locals and stack */
     for (; ref < frame->stackpointer; ref++) {
-        // This is a bit tricky! We want to ignore deferred references when
-        // computing the incoming references, but otherwise treat them like
-        // regular references.
-        if (PyStackRef_IsDeferred(*ref) &&
-            (visit == visit_decref || visit == visit_decref_unreachable)) {
-            continue;
+        if (_PyGC_VisitStackRef(ref, visit, arg) < 0) {
+            return -1;
         }
-        Py_VISIT(PyStackRef_AsPyObjectBorrow(*ref));
     }
     return 0;
 }

Python/generated_cases.c.h

@@ -1018,7 +1018,7 @@
             {
                 _PyInterpreterFrame *shim = _PyFrame_PushTrampolineUnchecked(
                     tstate, (PyCodeObject *)&_Py_InitCleanup, 1, frame);
-                assert(_PyCode_CODE((PyCodeObject *)shim->f_executable)[0].op.code == EXIT_INIT_CHECK);
+                assert(_PyCode_CODE(_PyFrame_GetCode(shim))[0].op.code == EXIT_INIT_CHECK);
                 /* Push self onto stack of shim */
                 shim->localsplus[0] = PyStackRef_DUP(self);
                 PyFunctionObject *init_func = (PyFunctionObject *)PyStackRef_AsPyObjectSteal(init);

Python/legacy_tracing.c

@@ -143,9 +143,8 @@ _PyEval_SetOpcodeTrace(
     bool enable
 ) {
     assert(frame != NULL);
-    assert(PyCode_Check(frame->f_frame->f_executable));
 
-    PyCodeObject *code = (PyCodeObject *)frame->f_frame->f_executable;
+    PyCodeObject *code = _PyFrame_GetCode(frame->f_frame);
     _PyMonitoringEventSet events = 0;
 
     if (_PyMonitoring_GetLocalEvents(code, PY_MONITORING_SYS_TRACE_ID, &events) < 0) {

Python/tracemalloc.c

@@ -250,7 +250,7 @@ hashtable_compare_traceback(const void *key1, const void *key2)
 static void
 tracemalloc_get_frame(_PyInterpreterFrame *pyframe, frame_t *frame)
 {
-    assert(PyCode_Check(pyframe->f_executable));
+    assert(PyStackRef_CodeCheck(pyframe->f_executable));
     frame->filename = &_Py_STR(anon_unknown);
     int lineno = PyUnstable_InterpreterFrame_GetLine(pyframe);
     if (lineno < 0) {
@@ -258,7 +258,7 @@ tracemalloc_get_frame(_PyInterpreterFrame *pyframe, frame_t *frame)
     }
     frame->lineno = (unsigned int)lineno;
 
-    PyObject *filename = filename = ((PyCodeObject *)pyframe->f_executable)->co_filename;
+    PyObject *filename = filename = _PyFrame_GetCode(pyframe)->co_filename;
 
     if (filename == NULL) {
 #ifdef TRACE_DEBUG