gh-117783: Immortalize objects that use deferred reference counting (#118112)

Deferred reference counting is not fully implemented yet. As a temporary
measure, we immortalize objects that would use deferred reference
counting to avoid multi-threaded scaling bottlenecks.

This is only performed in the free-threaded build once the first
non-main thread is started. Additionally, some tests, including refleak
tests, suppress this behavior.

1 files changed, 30 insertions, 0 deletions

diff --git a/Python/gc_free_threading.c b/Python/gc_free_threading.c
index 9cf0e989d09..8c0940d8f06 100644
--- a/Python/gc_free_threading.c
+++ b/Python/gc_free_threading.c
@@ -704,6 +704,12 @@ _PyGC_Init(PyInterpreterState *interp)
 {
     GCState *gcstate = &interp->gc;
 
+    if (_Py_IsMainInterpreter(interp)) {
+        // gh-117783: immortalize objects that would use deferred refcounting
+        // once the first non-main thread is created.
+        gcstate->immortalize.enable_on_thread_created = 1;
+    }
+
     gcstate->garbage = PyList_New(0);
     if (gcstate->garbage == NULL) {
         return _PyStatus_NO_MEMORY();
@@ -1781,6 +1787,30 @@ custom_visitor_wrapper(const mi_heap_t *heap, const mi_heap_area_t *area,
     return true;
 }
 
+// gh-117783: Immortalize objects that use deferred reference counting to
+// temporarily work around scaling bottlenecks.
+static bool
+immortalize_visitor(const mi_heap_t *heap, const mi_heap_area_t *area,
+                    void *block, size_t block_size, void *args)
+{
+    PyObject *op = op_from_block(block, args, false);
+    if (op != NULL && _PyObject_HasDeferredRefcount(op)) {
+        _Py_SetImmortal(op);
+        op->ob_gc_bits &= ~_PyGC_BITS_DEFERRED;
+    }
+    return true;
+}
+
+void
+_PyGC_ImmortalizeDeferredObjects(PyInterpreterState *interp)
+{
+    struct visitor_args args;
+    _PyEval_StopTheWorld(interp);
+    gc_visit_heaps(interp, &immortalize_visitor, &args);
+    interp->gc.immortalize.enabled = 1;
+    _PyEval_StartTheWorld(interp);
+}
+
 void
 PyUnstable_GC_VisitObjects(gcvisitobjects_t callback, void *arg)
 {