Re: [PATCH 00/16] mm: prepare for converting vm->vm_flags to 64-bit

Andrew Morton
2012-03-22T23:09:47Z

On Fri, 23 Mar 2012 02:52:05 +0400
Konstantin Khlebnikov <khlebnikov@openvz.org> wrote:

> >> Is this ok?
> >
> > Yes.
> >
> > The thing about __nocast is that it's so *very* very easy to lose it.
> > For example, do this:
> >
> > typedef long __nocast long_t;
> >
> > int main(long_t a)
> > {
> > return a;
> > }
> >
> > and you get the (expected) warning.
> >
> > HOWEVER. Now do "return a+1" instead, and the warning goes away. Why?
> > Because the expression ends up having just the type "long", because
> > the "a" mixed happily with the "1" (that was cast from 'int' to 'long'
> > by the normal C type rules).
> >
> > That is arguably a bug, but this kind of thing really wasn't what
> > __nocast was designed for. The __nocast design ended up being too
> > weak, though, and we hardly use it in the kernel.
> >
>
> Thanks. Looks like "__nocast" totally undocumented.
> It would be nice to add something about this into Documentation/sparse.txt

Yup, Chris has added this to his todo list (thanks!).