After a quick glance at the source code for the spidev library, I think it is unlikely but not impossible to be the home for the bug. It does do malloc() and free(), but only for data that is greater than 256 bytes. Short tx and rx data is kept in static local buffers. Also, these calls do not match the partial stack trace I got. There is a small amount of allocating and deallocating Python objects, however, including calls to PyList_New(), PyList_SET_ITEM(), and Py_TYPE(self)->tp_free((PyObject *)self), so it's possible that the bug is buried under one of those.