Crash report

What happened?

When cycling a lot of threads and modifying objects which use PyMem_FreeDelayed() in those threads the queues of memory blocks to free build up without ever being freed until the process runs out of memory, unless something else causes them to be freed. For example in the reproducer below commenting in the gc.collect() or the for loop will cause the delayed blocks to be freed correctly. Note that this is an extreme case.

Reproducer:

import gc
import threading


def fupmem(b, l):
    b.wait()
    l *= 2


def check(funcs, *args):
    barrier = threading.Barrier(len(funcs))
    thrds = []

    for func in funcs:
        thrd = threading.Thread(target=func, args=(barrier, *args))

        thrds.append(thrd)
        thrd.start()

    for thrd in thrds:
        thrd.join()


if __name__ == "__main__":
    count = 0

    while True:
        print(count := count + 1)

        check([fupmem] * 10, [None] * 256)

        # gc.collect()

        # for i in range(1000):
        #     l = []
        #     del l

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.14.0a5+ experimental free-threading build (heads/main:b3c18bfd828, Mar 3 2025, 09:13:46) [GCC 11.4.0]

Linked PRs

• gh-130794: Process interpreter QSBR queue in _PyMem_AbandonDelayed. #130808
• [3.13] gh-130794: Process interpreter QSBR queue in _PyMem_AbandonDelayed. (gh-130808) #130857