A context switch can happen between any two of those instructions. That means one thread could load the global, then another thread could load the same value, resulting in both of them writing back the same incremented value. Or, between opcodes 6 and 8 (between the lines of Python code), you could store the value, then fetch back a different value.
I see now. Then we can go with Steven's idea. Let's keep the changes in locals temporarily and when it yields or returns then modify the __statics__ member. And even if it's a generator it will stop iteration some time and if it doesn't then the member wasn't meant to be modified.