Re: [PATCH] mm/vmalloc: avoid bogus -Wmaybe-uninitialized warning

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Jun 18, 2019 at 09:40:28AM -0400, Joel Fernandes wrote:
> On Tue, Jun 18, 2019 at 5:27 AM Arnd Bergmann <arnd@arndb.de> wrote:
> >
> > gcc gets confused in pcpu_get_vm_areas() because there are too many
> > branches that affect whether 'lva' was initialized before it gets
> > used:
> >
> > mm/vmalloc.c: In function 'pcpu_get_vm_areas':
> > mm/vmalloc.c:991:4: error: 'lva' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> >     insert_vmap_area_augment(lva, &va->rb_node,
> >     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >      &free_vmap_area_root, &free_vmap_area_list);
> >      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > mm/vmalloc.c:916:20: note: 'lva' was declared here
> >   struct vmap_area *lva;
> >                     ^~~
> >
> > Add an intialization to NULL, and check whether this has changed
> > before the first use.
> >
> > Fixes: 68ad4a330433 ("mm/vmalloc.c: keep track of free blocks for vmap allocation")
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > ---
> >  mm/vmalloc.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index a9213fc3802d..42a6f795c3ee 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -913,7 +913,12 @@ adjust_va_to_fit_type(struct vmap_area *va,
> >         unsigned long nva_start_addr, unsigned long size,
> >         enum fit_type type)
> >  {
> > -       struct vmap_area *lva;
> > +       /*
> > +        * GCC cannot always keep track of whether this variable
> > +        * was initialized across many branches, therefore set
> > +        * it NULL here to avoid a warning.
> > +        */
> > +       struct vmap_area *lva = NULL;
> 
> Fair enough, but is this 5-line comment really needed here?
> 
How it is rewritten now, probably not. I would just set it NULL and
leave the comment, but that is IMHO. Anyway

Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

Thanks!

--
Vlad Rezki