linux-kernel - Re: [PATCH] shmem: avoid maybe-uninitialized warning

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Message-ID: <20161024203218.GF13148@dhcp22.suse.cz>
Date:   Mon, 24 Oct 2016 22:32:18 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] shmem: avoid maybe-uninitialized warning

On Mon 24-10-16 21:42:36, Arnd Bergmann wrote:
> On Monday, October 24, 2016 6:22:44 PM CEST Michal Hocko wrote:
> > On Mon 24-10-16 17:25:03, Arnd Bergmann wrote:
> > > After enabling -Wmaybe-uninitialized warnings, we get a false-postive
> > > warning for shmem:
> > > 
> > > mm/shmem.c: In function ‘shmem_getpage_gfp’:
> > > include/linux/spinlock.h:332:21: error: ‘info’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > 
> > Is this really a false positive? If we goto clear and then 
> >         if (sgp <= SGP_CACHE &&
> >             ((loff_t)index << PAGE_SHIFT) >= i_size_read(inode)) {
> >                 if (alloced) {
> > 
> > we could really take a spinlock on an unitialized variable. But maybe
> > there is something that prevents from that...
> 
> I did the patch a few weeks ago (I sent the more important
> ones out first) and I think I concluded then that 'alloced'
> would be false in that case.

OK, I guess you are right and alloced is set only after info has been
already initialized. So this really looks like a false positive.

> 
> > Anyway the whole shmem_getpage_gfp is really hard to follow due to gotos
> > and labels proliferation.
> 
> Exactly. Maybe we should mark the patch for -stable backports after all
> just to be sure.

I am not really sure a stable backport is really necessary but a cleanup
in this area would be more than welcome. At least from me ;)
-- 
Michal Hocko
SUSE Labs