8000 gh-117657: Fix race data race in `_Py_IsOwnedByCurrentThread()` by mpage · Pull Request #118258 · python/cpython · GitHub
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gh-117657: Fix race data race in _Py_IsOwnedByCurrentThread() #118258

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Merged
merged 3 commits into from
Apr 26, 2024
Merged

gh-117657: Fix race data race in _Py_IsOwnedByCurrentThread() #118258

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7581784
Fix race data race in `_Py_IsOwnedByCurrentThread()`
mpage Apr 24, 2024
425aa3f
Only use atomics with TSAN
mpage Apr 25, 2024
639c6bd
Merge branch 'main' into gh-117657-tsan-isownedby
mpage Apr 25, 2024
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Fix race data race in _Py_IsOwnedByCurrentThread() 
We need to load `ob_tid` atomically; it may be set to zero in another
thread without any intervening operations that establish a happens-before
relationship.

Using a relaxed load should be sufficient; we do not depend on the store
publishing any other information.

Examples:

The load in `_Py_IsOwnedByCurrentThread()`:

https://github.com/python/cpython/blob/4b10e209c76f9f36f8ae2e4d713b3a01591c1856/Include/object.h#L306

races with the store in `_Py_ExplicitMergeRefcount()` ([example](https://gist.github.com/mpage/0342ebce4b5b446689c41cdd72f16e61)):

https://github.com/python/cpython/blob/4b10e209c76f9f36f8ae2e4d713b3a01591c1856/Objects/object.c#L421

and the store in `_Py_MergeZeroLocalRefcount()` ([example](https://gist.github.com/mpage/eaec11eda6e2d988ed0dd2bcfd6d606b)):

https://github.com/python/cpython/blob/4b10e209c76f9f36f8ae2e4d713b3a01591c1856/Objects/object.c#L378
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@mpage
mpage committed Apr 24, 2024
commit 758178407de4d6f66ddfd1a3a580f8c480b27554
2 changes: 1 addition & 1 deletion Include/object.h
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Expand Up @@ -303,7 +303,7 @@ _Py_ThreadId(void)
static inline Py_ALWAYS_INLINE int
_Py_IsOwnedByCurrentThread(PyObject *ob)
{
return ob->ob_tid == _Py_ThreadId();
return _Py_atomic_load_uintptr_relaxed(&ob->ob_tid) == _Py_ThreadId();
}
#endif

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1 change: 0 additions & 1 deletion Tools/tsan/suppressions_free_threading.txt
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Expand Up @@ -15,7 +15,6 @@ race:_add_to_weak_set
race:_in_weak_set
race:_mi_heap_delayed_free_partial
race:_Py_IsImmortal
race:_Py_IsOwnedByCurrentThread
race:_PyEval_EvalFrameDefault
race:_PyFunction_SetVersion
race:_PyImport_AcquireLock
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0