From 48b6866047ab7d6001ff253053ee239ad2862277 Mon Sep 17 00:00:00 2001 From: Neil Schemenauer Date: Tue, 20 Jan 2026 10:01:09 -0800 Subject: [PATCH] gh-144054: no deferred refcount for untracked (gh-144081) This reverts gh-144055 and fixes the bug in a different way. Deferred reference counting relies on the object being tracked by the GC, otherwise the object will live until interpreter shutdown. So, take care that we do not enable deferred reference counting for objects that are untracked. Also, if a tuple has deferred reference counting enabled, don't untrack it. --- Python/gc_free_threading.c | 58 +++++++++++++++----------------------- Python/specialize.c | 2 +- 2 files changed, 24 insertions(+), 36 deletions(-) diff --git a/Python/gc_free_threading.c b/Python/gc_free_threading.c index beb3fa588f4..0ec9c58a792 100644 --- a/Python/gc_free_threading.c +++ b/Python/gc_free_threading.c @@ -308,18 +308,17 @@ disable_deferred_refcounting(PyObject *op) // should also be disabled when we turn off deferred refcounting. _PyObject_DisablePerThreadRefcounting(op); } - if (_PyObject_GC_IS_TRACKED(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); - } + + // 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); } } @@ -507,6 +506,10 @@ gc_visit_thread_stacks(PyInterpreterState *interp, struct collection_state *stat static bool gc_maybe_untrack(PyObject *op) { + if (_PyObject_HasDeferredRefcount(op)) { + // deferred refcounting only works if the object is tracked + return false; + } // Currently we only check for tuples containing only non-GC objects. In // theory we could check other immutable objects that contain references // to non-GC objects. @@ -1019,7 +1022,7 @@ update_refs(const mi_heap_t *heap, const mi_heap_area_t *area, } _PyObject_ASSERT(op, refcount >= 0); - if (refcount > 0 && !_PyObject_HasDeferredRefcount(op)) { + if (refcount > 0) { if (gc_maybe_untrack(op)) { gc_restore_refs(op); return true; @@ -1241,30 +1244,19 @@ scan_heap_visitor(const mi_heap_t *heap, const mi_heap_area_t *area, return true; } + 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); + } + // object is reachable, restore `ob_tid`; we're done with these objects gc_restore_tid(op); gc_clear_alive(op); return true; } -// Disable deferred refcounting for reachable objects during interpreter -// shutdown. This ensures that these objects are collected immediately when -// their last reference is removed. This needs to consider both tracked and -// untracked GC objects, since either might have deferred refcounts enabled. -static bool -scan_heap_disable_deferred(const mi_heap_t *heap, const mi_heap_area_t *area, - void *block, size_t block_size, void *args) -{ - PyObject *op = op_from_block_all_gc(block, args); - if (op == NULL) { - return true; - } - if (!_Py_IsImmortal(op)) { - disable_deferred_refcounting(op); - } - return true; -} - static int move_legacy_finalizer_reachable(struct collection_state *state); @@ -1499,10 +1491,6 @@ deduce_unreachable_heap(PyInterpreterState *interp, // Restores ob_tid for reachable objects. gc_visit_heaps(interp, &scan_heap_visitor, &state->base); - if (state->reason == _Py_GC_REASON_SHUTDOWN) { - gc_visit_heaps(interp, &scan_heap_disable_deferred, &state->base); - } - if (state->legacy_finalizers.head) { // There may be objects reachable from legacy finalizers that are in // the unreachable set. We need to mark them as reachable. diff --git a/Python/specialize.c b/Python/specialize.c index 432053f8522..845416a1d5b 100644 --- a/Python/specialize.c +++ b/Python/specialize.c @@ -362,7 +362,7 @@ static uint32_t function_get_version(PyObject *o, int opcode); static void maybe_enable_deferred_ref_count(PyObject *op) { - if (!_Py_IsOwnedByCurrentThread(op)) { + if (!_Py_IsOwnedByCurrentThread(op) && _PyObject_GC_IS_TRACKED(op)) { // For module level variables that are heavily used from multiple // threads, deferred reference counting provides good scaling // benefits. The downside is that the object will only be deallocated