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

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Date:   Mon, 13 Nov 2017 13:24:14 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Michal Hocko <mhocko@...nel.org>
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, Nov 13, 2017 at 12:54 PM, Michal Hocko <mhocko@...nel.org> wrote:
> 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.

That might also be useful. Some warnings in gcc get disabled when
they show up inside of a macro, and that could presumably be done
here too.

      Arnd