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

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Date:   Mon, 13 Nov 2017 12:43:08 +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 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

       Arnd