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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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3
Modules/_threadmodule.c
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@ -333,9 +333,6 @@ thread_run(void *boot_raw)
// _PyRuntimeState_SetFinalizing() is called. At this point, all Python
// threads must exit, except of the thread calling Py_Finalize() which
// holds the GIL and must not exit.
//
// At this stage, tstate can be a dangling pointer (point to freed memory),
// it's ok to call _PyThreadState_MustExit() with a dangling pointer.
if (_PyThreadState_MustExit(tstate)) {
// Don't call PyThreadState_Clear() nor _PyThreadState_DeleteCurrent().
// These functions are called on tstate indirectly by Py_Finalize()