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[3.14] gh-137433: Fix deadlock with stop-the-world and daemon threads (gh-137735) (GH-138965)

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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`.

(cherry picked from commit 90fe3250f8)

Co-authored-by: Sam Gross <colesbury@gmail.com>
This commit is contained in:
Miss Islington (bot) 2025-10-07 20:46:52 +02:00 • committed by GitHub
parent db8b943259
commit e09f33e5bf
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4 changed files with 51 additions and 4 deletions
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19
Python/parking_lot.c
View file

@ -342,7 +342,19 @@ _PyParkingLot_Park(const void *addr, const void *expected, size_t size,
enqueue(bucket, addr, &wait);
_PyRawMutex_Unlock(&bucket->mutex);
int res = _PySemaphore_Wait(&wait.sema, timeout_ns, detach);
PyThreadState *tstate = NULL;
if (detach) {
tstate = _PyThreadState_GET();
if (tstate && _PyThreadState_IsAttached(tstate)) {
// Only detach if we are attached
PyEval_ReleaseThread(tstate);
}
else {
tstate = NULL;
}
}
int res = _PySemaphore_Wait(&wait.sema, timeout_ns, 0);
if (res == Py_PARK_OK) {
goto done;
}
@ -354,7 +366,7 @@ _PyParkingLot_Park(const void *addr, const void *expected, size_t size,
// Another thread has started to unpark us. Wait until we process the
// wakeup signal.
do {
res = _PySemaphore_Wait(&wait.sema, -1, detach);
res = _PySemaphore_Wait(&wait.sema, -1, 0);
} while (res != Py_PARK_OK);
goto done;
}
@ -366,6 +378,9 @@ _PyParkingLot_Park(const void *addr, const void *expected, size_t size,
done:
_PySemaphore_Destroy(&wait.sema);
if (tstate) {
PyEval_AcquireThread(tstate);
}
return res;
}

6
Python/pystate.c
View file

@ -2345,13 +2345,15 @@ 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;
@ -2417,13 +2419,13 @@ 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