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SubjectRe: linux-next: build warning after merge of the akpm-current tree
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Hi,

On 2017/11/17 11:53, Stephen Rothwell wrote:
> 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):
>

I also came across this issue using Linus' tree and my gcc is gcc version 5.4.0 20160609
(Ubuntu/Linaro 5.4.0-6ubuntu1~16.04.5).

CC drivers/clk/renesas/rcar-gen3-cpg.o
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));
^
CC drivers/clk/renesas/renesas-cpg-mssr.o

Thanks,
Shaokun

> 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));
> ^
>

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