Serhiy is right about the theoretical concern here.  However, it's probably quite difficult to find a concrete situation where this occurs, because we're talking about mbuf_clear and the managerbuffer object can't really get involved in a reference cycle by itself (not in normal use anyway): the memoryview object does, but it's a different thing.

By the way: PyBuffer_Release() returns void, so IMO it's a bug if it can return with an exception set.  We should fix that rather than focus on mbuf_clear().