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GeezerVison is showing github.com — Inconsistency handling of immortal objects in gc · Issue #137110 · python/cpython

Inconsistency handling of immortal objects in gc #137110

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Inconsistency handling of immortal objects in gc#137110
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interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

Description

@sergey-miryanov
sergey-miryanov
opened on Jul 25, 2025
Issue body actions

Bug report

Bug description:

There is some inconsistency in handling of immortal objects in default-build's GC.

In update_refs immortal objects just untracked (all immortal objects that we face in GC is an accidentally immortalized).

cpython/Python/gc.c

Lines 495 to 500 in 1e69cd1

if (_Py_IsImmortal(op)) {
assert(!_Py_IsStaticImmortal(op));
_PyObject_GC_UNTRACK(op);
gc = next;
continue;
}

update_refs called for all collections (young, increment and full). But for incremental collection if increment_size < gcstate->work_to_do we steal some objects from neighbor gen and call expand_region_transitively_reachable where immortal objects moved to permanent generation:

cpython/Python/gc.c

Lines 1397 to 1402 in 1e69cd1

if (_Py_IsImmortal(op)) {
PyGC_Head *next = GC_NEXT(gc);
gc_list_move(gc, &gcstate->permanent_generation.head);
gc = next;
continue;
}

So, one part of immortal objects can be untracked, and other part can be moved to perm gen.

As I understand from #127519 it is preferring to untrack immortal objects in all cases.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

  • gh-137110: Untrack immortal objects from expand_region_transitivity_reachable #137111

Activity

  1. sergey-miryanov
    added
    type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error
    on Jul 25, 2025
  2. brianschubert
    added
    interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)
    on Jul 25, 2025

  3. sergey-miryanov commented on May 3, 2026

    @sergey-miryanov
    sergey-miryanov
    on May 3, 2026
    ContributorAuthor
    More actions

    Now that the generational GC has landed, I'll close this as obsolete.

  4. sergey-miryanov
    closed this as not plannedon May 3, 2026
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