Adding fine-grained locks without risking deadlock is pretty tricky. Perhaps too tricky for automatic addition. As I (shallowly) understand it, STM doesn't in

Adding fine-grained locks without risking deadlock is pretty tricky. Perhaps too tricky for automatic addition. As I (shallowly) understand it, STM doesn't introduce deadlock threats, so it requires less-amazing feats to add it automatically. Note, again, that the FA does admit that using STM will perform more poorly than fine-grained locking, and he only hopes that it will scale well enough that extra cores will pay for it. As I understand it, one of the reasons for the GIL is that it makes it easier to write C extensions (especially as compared with fine-grained locking, which is not friendly to extensions), and I'm not sure, but I think the STM scheme preserves this property?