Bug report

Bug description:

A data race exists in lists when they are concurrently updated and indexed (or otherwise accessed): for slice assignment and other ways of adding or removing multiple items (list.extend, in-place multiplication), memory writes to the newly allocated areas happen with memcpy/memmove, which have no write size/atomicity guarantees (and it's not unlikely that they will perform jagged writes). These reallocations happen with the lock held, but since we're relying on QSBR, concurrent reads of those memory areas are technically possible.

The tests in PR#128798 (test_free_threading.test_iteration), when run under ThreadSanitizer, expose those races. (They are suppressed in Tools/tsan/suppressions_free_threading.txt).

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-129069: make list ass_slice and memory_repeat safe in free-threading #131882