linux-kernel - Re: linux-next: build warning after merge of the akpm-current tree

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Message-ID: <20171113115430.pms47vgz5uszqsjm@dhcp22.suse.cz>
Date:   Mon, 13 Nov 2017 12:54:30 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: linux-next: build warning after merge of the akpm-current tree

On Mon 13-11-17 12:43:08, Arnd Bergmann wrote:
> On Mon, Nov 13, 2017 at 9:09 AM, Michal Hocko <mhocko@...nel.org> wrote:
> > On Mon 13-11-17 16:42:06, Stephen Rothwell wrote:
> >> Hi Andrew,
> >>
> >> After merging the akpm-current tree, today's linux-next build (powerpc
> >> ppc64_defconfig) produced this warning:
> >>
> >> In file included from include/linux/mmzone.h:17:0,
> >>                  from include/linux/mempolicy.h:10,
> >>                  from mm/mempolicy.c:70:
> >> mm/mempolicy.c: In function 'mpol_to_str':
> >> include/linux/nodemask.h:107:41: warning: the address of 'nodes' will always evaluate as 'true' [-Waddress]
> >>  #define nodemask_pr_args(maskp) (maskp) ? MAX_NUMNODES : 0, (maskp) ? (maskp)->bits : NULL
> >>                                          ^
> >> mm/mempolicy.c:2817:11: note: in expansion of macro 'nodemask_pr_args'
> >>            nodemask_pr_args(&nodes));
> >>            ^
> >
> > Hmm, this warning is quite surprising to me. Sure in this particular
> > case maskp will always be non-NULL so we always expand to
> >         MAX_NUMNODES, maskp->bits
> > which is what we want. But we have other users which may be NULL. Does
> > anybody understan why this warns at all?
> 
> As I understand it, the warning tries to address a common typo of accidentally
> testing the pointer to a stack object for being non-NULL, rather than the object
> pointed to for being non-zero.
> 
> Adding an extra '!= NULL' comparison gets rid of the warning for me:
> 
> #define nodemask_pr_args(maskp)  \
>    ((maskp) != NULL) ? MAX_NUMNODES : 0, \
>    ((maskp) != NULL) ?(maskp)->bits : NULL

OK, that is a reasonable workaround. I was talking to our gcc guy and
he suggested to report a bug for this. Andrew, could you fold the
explicit != NULL check into the patch please?

-- 
Michal Hocko
SUSE Labs