Re: [linux-next:master 9529/10007] mm/hugetlb.c:1591:9: warning: no previous prototype for function 'hugetlb_basepage_index'

[Matthew Wilcox <willy@infradead.org>]

On Tue, Jun 15, 2021 at 11:50:40AM -0700, Nick Desaulniers wrote:
> On Tue, Jun 15, 2021 at 11:32 AM Nathan Chancellor <nathan@kernel.org> wrote:
> >
> > On Tue, Jun 15, 2021 at 05:03:40PM +0100, Matthew Wilcox wrote:
> > > On Tue, Jun 15, 2021 at 11:58:36PM +0800, kernel test robot wrote:
> > > >         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=arm64
> > > >
> > > > If you fix the issue, kindly add following tag as appropriate
> > > > Reported-by: kernel test robot <lkp@intel.com>
> > > >
> > > > All warnings (new ones prefixed by >>):
> > > >
> > > > >> mm/hugetlb.c:1591:9: warning: no previous prototype for function 'hugetlb_basepage_index' [-Wmissing-prototypes]
> > > >    pgoff_t hugetlb_basepage_index(struct page *page)
> > >
> > > So clang requires the prototype to still be in scope, while gcc doesn't.
> > > Does one of our clangers want to file a bug about that?
> >
> > I see the exact same warning with GCC 11.1.0:
> >
> > $ curl -LSs https://lore.kernel.org/linux-mm/202106152328.Mh5S48hE-lkp@intel.com/2-a.bin | gzip -d > .config
> >
> > $ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux- W=1 olddefconfig mm/hugetlb.c
> > mm/hugetlb.c:1591:9: warning: no previous prototype for 'hugetlb_basepage_index' [-Wmissing-prototypes]
> >  1591 | pgoff_t hugetlb_basepage_index(struct page *page)
> >       |         ^~~~~~~~~~~~~~~~~~~~~~
> 
> Since this is a commonly recurring warning for W=1 builds, then this
> function either should be declared as having static linkage if its
> uses are local to the same file, or a prototype should be declared in
> a header so that callers and callee agree on function signature.

Oh, you haven't understood the problem.

static inline int bar(void)
{
        int foo(void);

        return foo();
}

int foo(void) { return 1; }

The prototype isn't _missing_.  It's just no longer in scope.

Since gcc and clang behave the same way here, we should adjust the source
to make foo visible outside bar.  But this is a case where both compilers
are wrong.