Although the core objects won't be affected by this change because of the interpreter lock protection, we have no provisions so far for custom object's thread safety.

If I understand your concern correctly, you are worried that somebody uses your allocator without holding the interpreter lock. I think it is *extremely* unlikely that a module author will use any Py* function or macro while not holding the lock. I've analyzed a few freely-available extension modules in this respect, and found no occurence of such code. The right way is to document that restriction, both in NEWS and in the C API documentation, and accept the unlikely chance of breaking something. Regards, Martin