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GeezerVison is showing github.com — gh-151475: Fix data race in faulthandler watchdog on free-threaded builds by timurmamedov1 · Pull Request #152123 · python/cpython

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@timurmamedov1

@timurmamedov1 timurmamedov1 commented Jun 24, 2026

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dump_traceback_later() and cancel_dump_traceback_later() mutate the watchdog state in _PyRuntime.faulthandler.thread without synchronization. On free-threaded builds, concurrent calls corrupt the cancel_event/running lock handshake, since two threads can both allocate cancel_event (leaking one lock), or release a lock they don't hold, causing a fatal abort.

Fix by adding a PyMutex to _faulthandler_runtime_state and acquiring it in both dump_traceback_later and cancel_dump_traceback_later entry points. The mutex is acquired after argument validation (which may call into Python) but before any shared state access.

These are not hot paths and none run in signal-handler context, so a simple mutex is sufficient.

  • Issue: faulthandler: data races in enable()/disable() and dump_traceback_later() under free threading #151475

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Copilot AI review requested due to automatic review settings June 24, 2026 19:57
Copilot AI reviewed Jun 24, 2026

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@bedevere-app bedevere-app Bot mentioned this pull request Jun 24, 2026
faulthandler: data races in enable()/disable() and dump_traceback_later() under free threading #151475
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@bedevere-app bedevere-app Bot added the awaiting review label Jun 24, 2026
@timurmamedov1
pythongh-151475: Fix data race in faulthandler watchdog on free-threa…
5014d9f 
…ded builds

Add a PyMutex to serialize dump_traceback_later() and
cancel_dump_traceback_later() calls.  Without this, concurrent
arm/cancel from multiple threads corrupts the cancel_event/running
lock handshake, causing an abort from unlocking an unheld lock.
@timurmamedov1
timurmamedov1 force-pushed the gh-151475-fix-faulthandler-thread-safety branch from 1923738 to 5014d9f Compare June 24, 2026 20:00
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github-actions Bot commented Sep 23, 2026

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This PR is stale because it has been open for 90 days with no activity.
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@github-actions github-actions Bot added the stale Stale PR or inactive for long period of time. label Sep 23, 2026
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@timurmamedov1
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