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

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Date:   Wed, 19 Jun 2019 12:36:36 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Roman Penyaev <rpenyaev@...e.de>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...e.hu>, Tejun Heo <tj@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/vmalloc: avoid bogus -Wmaybe-uninitialized warning

On Tue, Jun 18, 2019 at 01:59:20PM -0700, Andrew Morton wrote:
> On Tue, 18 Jun 2019 16:06:22 +0200 Uladzislau Rezki <urezki@...il.com> wrote:
> 
> > 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@...db.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@...db.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
> > 
> 
> I agree - given that the patch does this:
> 
> @@ -972,7 +977,7 @@ adjust_va_to_fit_type(struct vmap_area *
>  	if (type != FL_FIT_TYPE) {
>  		augment_tree_propagate_from(va);
>  
> -		if (type == NE_FIT_TYPE)
> +		if (lva)
>  			insert_vmap_area_augment(lva, &va->rb_node,
>  				&free_vmap_area_root, &free_vmap_area_list);
>  	}
> 
> the comment simply isn't relevant any more.  Although I guess this
> might be a bit helpful:
> 
> @@ -977,7 +972,7 @@ adjust_va_to_fit_type(struct vmap_area *
>  	if (type != FL_FIT_TYPE) {
>  		augment_tree_propagate_from(va);
>  
> -		if (lva)
> +		if (lva)	/* type == NE_FIT_TYPE */
>  			insert_vmap_area_augment(lva, &va->rb_node,
>  				&free_vmap_area_root, &free_vmap_area_list);
>  	}
> 
That comment makes it much clear, thanks!

--
Vlad Rezki