# Crash report ### What happened? Trying to iterate on a generator from multiple threads under the free-threaded build results in a segmentation fault. Here is a minimal repro: ```python import concurrent.futures def gen(): while True: yield it = gen() with concurrent.futures.ThreadPoolExecutor() as executor: while True: _ = executor.submit(lambda: next(it)) ``` The issue seems to be specific to generators as other kinds of iterators work well in parallel. ### CPython versions tested on: CPython main branch ### Operating systems tested on: Linux ### Output from running 'python -VV' on the command line: Python 3.14.0a0 experimental free-threading build (heads/main:c3b6dbff2c, Jun 10 2024, 16:54:16) [GCC 11.4.0] <!-- gh-linked-prs --> ### Linked PRs * gh-120327 * gh-142599 * gh-142995 * gh-143022 * gh-143112 * gh-143128 <!-- /gh-linked-prs -->