Crash on 3.14.6 free threading

[ddorian]

I hit this when trying free-threading with playwright in microsoft/playwright-python#3123.

This happens also with gil=on.

Is this valid? Can I try to make a pull request?

❯ uv run --no-project --python 3.14.6t repro_min.py
python 3.14.6  gil_enabled=False
Fatal Python error: PyMutex_Unlock: unlocking mutex that is not locked
Python runtime state: initialized

Stack (most recent call first):
  File "/home/guru/Desktop/repro_min.py", line 39 in g2_body

Extension modules: greenlet._greenlet (total: 1)

script:

# /// script
# requires-python = "==3.14.6"
# dependencies = ["greenlet==3.5.2"]
# ///
# Run on free-threaded CPython:  uv run --no-project --python 3.14t repro_min.py
#
# greenlet never saves/restores CPython's per-thread critical-section pointer
# (tstate->critical_section) across a stack switch. Two greenlets that each
# switch *while holding a list.sort() critical section* interleave non-nested,
# corrupting the chain; the next blocking open() then aborts in
# _PyCriticalSection_SuspendAll with "PyMutex_Unlock: unlocking mutex that is
# not locked".
import sys

import greenlet

print(f"python {sys.version.split()[0]}  gil_enabled={sys._is_gil_enabled()}", flush=True)

main = greenlet.getcurrent()


def k1(x):
    g2.switch()  # into G2 while holding G1's sort critical section
    return x


def k2(x):
    g1.switch()  # back into G1 while holding G2's sort critical section
    return x


def g1_body():
    sorted([0], key=k1)
    g2.switch()


def g2_body():
    sorted([0], key=k2)
    open("/etc/hostname").read()  # SuspendAll over a stale, unlocked mutex -> fatal


g1 = greenlet.greenlet(g1_body)
g2 = greenlet.greenlet(g2_body)
g1.switch()
print("DONE no crash")

[jamadden]

Thanks! This is definitely a valid report, and you were exactly correct about the cause.

Thanks also for your offer to create a PR. While investigating the issue, I actually wound up patching greenlet myself as it seems to be a trivial fix. I very slightly adapted your sample code into the new test case (credited to you).