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

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Date:   Fri, 17 Nov 2017 10:56:34 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Stephen Rothwell <sfr@...b.auug.org.au>
Cc:     Michal Hocko <mhocko@...nel.org>,
        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 Fri, Nov 17, 2017 at 4:53 AM, Stephen Rothwell <sfr@...b.auug.org.au> wrote:
> Hi all,
>
> On Fri, 17 Nov 2017 09:44:39 +1100 Stephen Rothwell <sfr@...b.auug.org.au> wrote:
>>
>> On Mon, 13 Nov 2017 12:43:08 +0100 Arnd Bergmann <arnd@...db.de> 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:
>> > >>
>> > >> 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):

Right, I see now that all versions from gcc-4.6 to gcc-6 are affected
by this, while 4.5 and
earlier as well as 7 and 8 are not.

I'll try to come up with an alternative workaround, it will probably
be even uglier.

       Arnd