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PyList_SetItem missing atomic store #129643

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PyList_SetItem missing atomic store#129643
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interpreter-core(Objects, Python, Grammar, and Parser dirs)topic-free-threadingtype-bugAn unexpected behavior, bug, or error

Description

@kumaraditya303
kumaraditya303
opened on Feb 4, 2025

Bug report

PyList_SetItem currently uses Py_XSETREF to set the item and decref the old one, however the store is not atomic as such it can race with a concurrent read. The fix is to use a atomic store with release order to correctly set the new item and then decref the old object.

cc @colesbury @Yhg1s

Linked PRs

Activity

  1. kumaraditya303
    added
    topic-free-threading
    type-bugAn unexpected behavior, bug, or error
    on Feb 4, 2025
  2. picnixz
    added
    interpreter-core(Objects, Python, Grammar, and Parser dirs)
    on Feb 4, 2025

  3. colesbury commented on Feb 4, 2025

    @colesbury
    colesbury
    on Feb 4, 2025
    Contributor

    There is a similar issue in the function ins1:

    cpython/Objects/listobject.c

    Lines 467 to 470 in 979d766

    items = self->ob_item;
    for (i = n; --i >= where; )
    items[i+1] = items[i];
    items[where] = Py_NewRef(v);

  4. kumaraditya303
    added 3 commits that reference this issue on Feb 5, 2025

    gh-129643: fix thread safety of `PyList_SetItem` (#129644)

    fb5d1c9

    pythongh-129643: fix thread safety of `PyList_SetItem` (python#129644)

    d9c008a

    [3.13] gh-129643: fix thread safety of `PyList_SetItem` (#129644) (#1…

    b081091
  5. sobolevn
    added 4 commits that reference this issue on Feb 5, 2025

    pythongh-129643: Fix `PyList_Insert` in free-threading builds

    9ef7a2b

    gh-129643: Fix `PyList_Insert` in free-threading builds (#129680)

    63f0406

    [3.13] pythongh-129643: Fix `PyList_Insert` in free-threading builds (p…

    2b888d4

    [3.13] gh-129643: Fix `PyList_Insert` in free-threading builds (GH-12…

    8529d64
  6. sobolevn
    closed this as completedon Feb 6, 2025
  7. srinivasreddy
    added 2 commits that reference this issue on Feb 7, 2025

    pythongh-129643: fix thread safety of `PyList_SetItem` (python#129644)

    637383e

    pythongh-129643: Fix `PyList_Insert` in free-threading builds (python…

    ef4f829
  8. cmaloney
    added 2 commits that reference this issue on Feb 8, 2025

    pythongh-129643: fix thread safety of `PyList_SetItem` (python#129644)

    20272e9

    pythongh-129643: Fix `PyList_Insert` in free-threading builds (python…

    e512746
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