Is there a more efficient threading lock?

[Skip Montanaro]

> And yet, it appears that *something* changed between Python 2 and Python
3 such that it *is* atomic:

I haven't looked, but something to check in the source is opcode
prediction. It's possible that after the BINARY_OP executes, opcode
prediction jumps straight to the STORE_FAST opcode, avoiding the transfer
to the top of the virtual machine loop. That would (I think) avoid checks
related to GIL release and thread switches.

I don't guarantee that's what's going on, and even if I'm correct, I don't
think you can rely on it.

Skip