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gh-124878: Fix race conditions during interpreter finalization (#130649)

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The PyThreadState field gains a reference count field to avoid
issues with PyThreadState being a dangling pointer to freed memory.
The refcount starts with a value of two: one reference is owned by the
interpreter's linked list of thread states and one reference is owned by
the OS thread. The reference count is decremented when the thread state
is removed from the interpreter's linked list and before the OS thread
calls `PyThread_hang_thread()`. The thread that decrements it to zero
frees the `PyThreadState` memory.

The `holds_gil` field is moved out of the `_status` bit field, to avoid
a data race where on thread calls `PyThreadState_Clear()`, modifying the
`_status` bit field while the OS thread reads `holds_gil` when
attempting to acquire the GIL.

The `PyThreadState.state` field now has `_Py_THREAD_SHUTTING_DOWN` as a
possible value. This corresponds to the `_PyThreadState_MustExit()`
check. This avoids race conditions in the free threading build when
checking `_PyThreadState_MustExit()`.
This commit is contained in:
Sam Gross 2025-03-06 10:38:34 -05:00 • committed by GitHub
parent c6dd2348ca
commit 052cb717f5
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13 changed files with 109 additions and 81 deletions
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19
Python/pylifecycle.c
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@ -2036,18 +2036,23 @@ _Py_Finalize(_PyRuntimeState *runtime)
// XXX Call something like _PyImport_Disable() here?
/* Destroy the state of all threads of the interpreter, except of the
/* Remove the state of all threads of the interpreter, except for the
current thread. In practice, only daemon threads should still be alive,
except if wait_for_thread_shutdown() has been cancelled by CTRL+C.
Clear frames of other threads to call objects destructors. Destructors
will be called in the current Python thread. Since
_PyRuntimeState_SetFinalizing() has been called, no other Python thread
can take the GIL at this point: if they try, they will exit
immediately. We start the world once we are the only thread state left,
We start the world once we are the only thread state left,
before we call destructors. */
PyThreadState *list = _PyThreadState_RemoveExcept(tstate);
for (PyThreadState *p = list; p != NULL; p = p->next) {
_PyThreadState_SetShuttingDown(p);
}
_PyEval_StartTheWorldAll(runtime);
_PyThreadState_DeleteList(list);
/* Clear frames of other threads to call objects destructors. Destructors
will be called in the current Python thread. Since
_PyRuntimeState_SetFinalizing() has been called, no other Python thread
can take the GIL at this point: if they try, they will hang in
_PyThreadState_HangThread. */
_PyThreadState_DeleteList(list, /*is_after_fork=*/0);
/* At this point no Python code should be running at all.
The only thread state left should be the main thread of the main