Hi David,

[auto build test WARNING on net-next/master]
[also build test WARNING on next-20170817]
[cannot apply to v4.13-rc5]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/David-Lamparter/bridge-learn-dst-metadata-in-FDB/20170820-001849
config: s390-allmodconfig (attached as .config)
compiler: s390x-linux-gnu-gcc (Debian 6.1.1-9) 6.1.1 20160705
reproduce:
        wget https://raw.githubusercontent.com/01org/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        make.cross ARCH=s390 

Note: it may well be a FALSE warning. FWIW you are at least aware of it now.
http://gcc.gnu.org/wiki/Better_Uninitialized_Warnings

All warnings (new ones prefixed by >>):

   In file included from include/uapi/linux/stddef.h:1:0,
                    from include/linux/stddef.h:4,
                    from include/uapi/linux/posix_types.h:4,
                    from include/uapi/linux/types.h:13,
                    from include/linux/types.h:5,
                    from net/mpls/af_mpls.c:1:
   net/mpls/af_mpls.c: In function 'mpls_rt_xmit':
>> include/linux/compiler.h:239:26: warning: 'out_dev' may be used uninitialized in this function [-Wmaybe-uninitialized]
     case 8: *(__u64 *)res = *(volatile __u64 *)p; break;  \
                             ^
   net/mpls/af_mpls.c:384:21: note: 'out_dev' was declared here
     struct net_device *out_dev;
                        ^~~~~~~
--
   In file included from include/uapi/linux/stddef.h:1:0,
                    from include/linux/stddef.h:4,
                    from include/uapi/linux/posix_types.h:4,
                    from include/uapi/linux/types.h:13,
                    from include/linux/types.h:5,
                    from net//mpls/af_mpls.c:1:
   net//mpls/af_mpls.c: In function 'mpls_rt_xmit':
>> include/linux/compiler.h:239:26: warning: 'out_dev' may be used uninitialized in this function [-Wmaybe-uninitialized]
     case 8: *(__u64 *)res = *(volatile __u64 *)p; break;  \
                             ^
   net//mpls/af_mpls.c:384:21: note: 'out_dev' was declared here
     struct net_device *out_dev;
                        ^~~~~~~

vim +/out_dev +239 include/linux/compiler.h

230fa253 Christian Borntraeger 2014-11-25  232  
d976441f Andrey Ryabinin       2015-10-19  233  #define __READ_ONCE_SIZE						\
d976441f Andrey Ryabinin       2015-10-19  234  ({									\
d976441f Andrey Ryabinin       2015-10-19  235  	switch (size) {							\
d976441f Andrey Ryabinin       2015-10-19  236  	case 1: *(__u8 *)res = *(volatile __u8 *)p; break;		\
d976441f Andrey Ryabinin       2015-10-19  237  	case 2: *(__u16 *)res = *(volatile __u16 *)p; break;		\
d976441f Andrey Ryabinin       2015-10-19  238  	case 4: *(__u32 *)res = *(volatile __u32 *)p; break;		\
d976441f Andrey Ryabinin       2015-10-19 @239  	case 8: *(__u64 *)res = *(volatile __u64 *)p; break;		\
d976441f Andrey Ryabinin       2015-10-19  240  	default:							\
d976441f Andrey Ryabinin       2015-10-19  241  		barrier();						\
d976441f Andrey Ryabinin       2015-10-19  242  		__builtin_memcpy((void *)res, (const void *)p, size);	\
d976441f Andrey Ryabinin       2015-10-19  243  		barrier();						\
d976441f Andrey Ryabinin       2015-10-19  244  	}								\
d976441f Andrey Ryabinin       2015-10-19  245  })
d976441f Andrey Ryabinin       2015-10-19  246  

:::::: The code at line 239 was first introduced by commit
:::::: d976441f44bc5d48635d081d277aa76556ffbf8b compiler, atomics, kasan: Provide READ_ONCE_NOCHECK()

:::::: TO: Andrey Ryabinin <aryabinin@virtuozzo.com>
:::::: CC: Ingo Molnar <mingo@kernel.org>

---
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