[BUG] _PyObject_GetDictPtr used by cython is not safe in free-threading

[kumaraditya303]

Describe the bug

Cython uses _PyObject_GetDictPtr in __Pyx_PyObject_GetMethod and reads the dict pointer directly. In free-threading the dict pointer is stored atomically as such it will lead to data races when dict is changed concurrently on the object.

cython/Cython/Utility/ObjectHandling.c

Line 1729 in c00a9c9

dictptr = _PyObject_GetDictPtr(obj);

Code to reproduce the behaviour:

No response

Expected behaviour

No data races

OS

Linux

Python version

3.14

Cython version

main

Additional context

See _PyObject_SetManagedDict in CPython for the use of atomic for setting the dict.

[da-woods]

Thanks. To me this looks like we just have to read from the pointer with an atomic load (ideally an acquire load I think). So this probably isn't a big change - just a case of making sure we're explicit about the atomic operation.

[kumaraditya303]

IIUC Cython implements __Pyx_PyObject_GetMethod for performance right? In that case why not call the function from CPython rather than implementing it here? The current version in CPython uses stackref so would scale better on free-threading.

[da-woods]

I'll need to investigate properly but: there's a few different uses of it. They're mostly for older versions of Python without vectorcall. For modern Python we use PyObject_VectorcallMethod instead (which I think is what you recommend).

It looks like there's somewhere that we always use it in the coroutine code. That's probably worth replacing with PyObject_VectorcallMethod.

If that's correct, it may be almost unused on recent Python.

[da-woods]

For 3.2 both parts of the fix have been merged and we'll only using this on older (non-freethreading) builds