Data race: sys.addaudithook() lazily creates interp->audit_hooks with no lock

Bug report

Full report: https://github.com/proxy/gist.github.com/devdanzin/0b13838fd6089e73a3f063ed8f68e733

Repro:

import sys, threading
assert not sys._is_gil_enabled(), "run free-threaded: PYTHON_GIL=0"

NADD = 24      # threads slamming the first-time lazy-init store (write @540) at once
NAUD = 8       # threads spinning audit events (should_audit read @239)
barrier = threading.Barrier(NADD + NAUD)

def _hook(*a):
    return None

def adder():
    barrier.wait()
    for _ in range(200):
        sys.addaudithook(_hook)          # write interp->audit_hooks (first time) @540

def auditor():
    barrier.wait()
    for _ in range(200000):
        sys.audit("fusil.tsan.test")     # should_audit read of interp->audit_hooks @239

ts = [threading.Thread(target=adder) for _ in range(NADD)]
ts += [threading.Thread(target=auditor) for _ in range(NAUD)]
for t in ts: t.start()
for t in ts: t.join()
print("done, no crash")

The fix is incoming.

Linked PRs

pythongh-154431: Fix data race in `sys.audithook`

gh-154431: Fix data race in `sys.audithook` (#154462)

[3.15] gh-154431: Fix data race in `sys.audithook` (GH-154462) (#154477)

gh-154431: Revert "gh-154431: Fix data race in `sys.audithook` (#154462…

The commit caused a refleak that made tier 1 buildbot fail (e.g. [here](e.g. https://buildbot.python.org/#/builders/1610/builds/3726)); it was failing throughout the weekend. I've reverted to get the CI green.
Please add the change itself (an un-revert) to the follow-up PR(s). Let me know if you want assistance.

Merge branch 'main' into issues/pythongh-154431-sys-addaudithook-race

[3.15] gh-154431: Revert "gh-154431: Fix data race in `sys.audithook` (…

pythongh-154431: Revert "pythongh-154431: Fix data race in `sys.audit…

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Data race: `sys.addaudithook()` lazily creates `interp->audit_hooks` with no lock #154431

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Data race: sys.addaudithook() lazily creates interp->audit_hooks with no lock #154431

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encukou commented on Jul 27, 2026

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Data race: `sys.addaudithook()` lazily creates `interp->audit_hooks` with no lock #154431