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SubjectRe: 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@kernel.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

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