Probably a mix of both; when developing a kernel, you constantly fight against the compiler trying to be smart and optiziming things out of your code, inlining

Probably a mix of both; when developing a kernel, you constantly fight against the compiler trying to be smart and optiziming things out of your code, inlining things, caching data, etc. In this case, it might be necessary to do a volatile read in case that flag has changed, forcing the compiler to reload it.