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gh-132070: Fix PyObject_Realloc thread-safety in free threaded Python - #143441
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…Python The PyObject header reference count fields must be initialized using atomic operations because they may be concurrently read by another thread (e.g., from _Py_TryIncref).
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Yhg1s
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| for (size_t i = 0; i != offset; i += sizeof(void*)) { | ||
| void *word; | ||
| memcpy(&word, (char*)ptr + i, sizeof(void*)); | ||
| _Py_atomic_store_ptr_relaxed((void**)((char*)newp + i), word); |
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Is there a reason for the intermediate word? And why the memcpy instead of just a cast + deref + assignment? Can't this just be:
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| _Py_atomic_store_ptr_relaxed((void**)((char*)newp + i), word); | |
| _Py_atomic_store_ptr_relaxed((void**)((char*)newp + i), (void *)((char *)ptr + 1)); |
(If it can't be, it probably deserves a comment.)
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My thinking was that the memcpy avoids accessing data the pointers of an incorrect size or strict aliasing issues. For example, we read ob_flags, ob_mutex, ob_gc_bits, and ob_ref_local together as a 64-bit load (on 64-bit systems).
I guess there's still the same issue when we write the data with _Py_atomic_store_ptr_relaxed, so maybe it doesn't matter.
From what I can tell, UBSan and ASan don't check for this kind of thing.
Co-authored-by: T. Wouters <thomas@python.org>
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…Python (pythongh-143441) The PyObject header reference count fields must be initialized using atomic operations because they may be concurrently read by another thread (e.g., from `_Py_TryIncref`).
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The PyObject header reference count fields must be initialized using atomic operations because they may be concurrently read by another thread (e.g., from
_Py_TryIncref).