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Date:   Fri, 17 Nov 2017 14:53:20 +1100
From:   Stephen Rothwell <sfr@canb.auug.org.au>
To:     Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Linux-Next Mailing List <linux-next@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: linux-next: build warning after merge of the akpm-current tree
Message-ID: <20171117145320.714972ca@canb.auug.org.au>
In-Reply-To: <20171117094439.5287c715@canb.auug.org.au>
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        <20171117094439.5287c715@canb.auug.org.au>
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Hi all,

On Fri, 17 Nov 2017 09:44:39 +1100 Stephen Rothwell <sfr@canb.auug.org.au> wrote:
>
> On Mon, 13 Nov 2017 12:43:08 +0100 Arnd Bergmann <arnd@arndb.de> wrote:
> >
> > On Mon, Nov 13, 2017 at 9:09 AM, Michal Hocko <mhocko@kernel.org> wrote:  
> > > On Mon 13-11-17 16:42:06, Stephen Rothwell wrote:    
> > >>
> > >> 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
> > 
> >        Arnd  
> 
> This warning now exists in Linus' tree :-(

Looking closer, it seems that the above workaround doesn't work for my
compiler (gcc v5.2.0):

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:108:11: warning: the comparison will always evaluate as 'true' for the address of 'nodes' will never be NULL [-Waddress]
  ((maskp) != NULL) ? MAX_NUMNODES : 0,  \
           ^
mm/mempolicy.c:2817:11: note: in expansion of macro 'nodemask_pr_args'
           nodemask_pr_args(&nodes));
           ^
include/linux/nodemask.h:109:11: warning: the comparison will always evaluate as 'true' for the address of 'nodes' will never be NULL [-Waddress]
  ((maskp) != NULL) ? (maskp)->bits : NULL
           ^
mm/mempolicy.c:2817:11: note: in expansion of macro 'nodemask_pr_args'
           nodemask_pr_args(&nodes));
           ^

-- 
Cheers,
Stephen Rothwell

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