On Fri, 28 May 2010 07:53:13 -0300 Cesar Eduardo Barros <cesarb@xxxxxxxxxx> wrote:
> kunmap_atomic() is currently at level -4 on Rusty's "Hard To Misuse"
> list[1] ("Follow common convention and you'll get it wrong"), except in
> some architectures when CONFIG_DEBUG_HIGHMEM is set[2][3].
>
> kunmap() takes a pointer to a struct page; kunmap_atomic(), however,
> takes takes a pointer to within the page itself. This seems to once in a
> while trip people up (the convention they are following is the one from
> kunmap()).
>
> Make it much harder to misuse, by moving it to level 9 on Rusty's
> list[4] ("The compiler/linker won't let you get it wrong"). This is done
> by refusing to build if the pointer passed to it is convertible to a
> struct page * but it is not a void * (verified by trying to convert it
> to a pointer to a dummy struct).
>
> The real kunmap_atomic() is renamed to kunmap_atomic_notypecheck()
> (which is what you would call in case for some strange reason calling it
> with a pointer to a struct page is not incorrect in your code).
>
Fair enough, that's a 99% fix. A long time ago I made kmap_atomic()
return a char * (iirc) and kunmap_atomic() is passed a char*. It
worked, but I ended up throwing it away. I don't precisely remember
why - I think it was intrusiveness and general hassle rather than
anything fundamental.
>
> ...
>
> +/* Prevent people trying to call kunmap_atomic() as if it were kunmap() */
> +struct __kunmap_atomic_dummy {};
> +#define kunmap_atomic(addr, idx) do { \
> + BUILD_BUG_ON( \
> + __builtin_types_compatible_p(typeof(addr), struct page *) && \
> + !__builtin_types_compatible_p(typeof(addr), struct __kunmap_atomic_dummy *)); \
> + kunmap_atomic_notypecheck((addr), (idx)); \
> + } while (0)
<looks around>
OK, it seems that __builtin_types_compatible_p() is supported on all
approved gcc versions.
We have a little __same_type() helper for this. __must_be_array()
should be using it, too.
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