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gh-137433: Fix deadlock with stop-the-world and daemon threads (gh-137735)

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There was a deadlock originally seen by Memray when a daemon thread
enabled or disabled profiling while the interpreter was shutting down.
I think this could also happen with garbage collection, but I haven't
seen that in practice.

The daemon thread could be hung while trying acquire the global rwmutex
that prevents overlapping global and per-interpreter stop-the-world events.
Since it already held the main interpreter's stop-the-world lock, it
also deadlocked the main thread, which is trying to perform interpreter
finalization.

Swap the order of lock acquisition to prevent this deadlock.
Additionally, refactor `_PyParkingLot_Park` so that the global buckets
hashtable is left in a clean state if the thread is hung in
`PyEval_AcquireThread`.
This commit is contained in:
Sam Gross 2025-09-16 09:21:58 +01:00 • committed by GitHub
parent 4c0d7bc52a
commit 90fe3250f8
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6 changed files with 55 additions and 31 deletions
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6
Python/pystate.c
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@ -2253,13 +2253,15 @@ stop_the_world(struct _stoptheworld_state *stw)
{
_PyRuntimeState *runtime = &_PyRuntime;
PyMutex_Lock(&stw->mutex);
// gh-137433: Acquire the rwmutex first to avoid deadlocks with daemon
// threads that may hang when blocked on lock acquisition.
if (stw->is_global) {
_PyRWMutex_Lock(&runtime->stoptheworld_mutex);
}
else {
_PyRWMutex_RLock(&runtime->stoptheworld_mutex);
}
PyMutex_Lock(&stw->mutex);
HEAD_LOCK(runtime);
stw->requested = 1;
@ -2325,13 +2327,13 @@ start_the_world(struct _stoptheworld_state *stw)
}
stw->requester = NULL;
HEAD_UNLOCK(runtime);
PyMutex_Unlock(&stw->mutex);
if (stw->is_global) {
_PyRWMutex_Unlock(&runtime->stoptheworld_mutex);
}
else {
_PyRWMutex_RUnlock(&runtime->stoptheworld_mutex);
}
PyMutex_Unlock(&stw->mutex);
}
#endif // Py_GIL_DISABLED