[rcu:ms 7/8] include/linux/mm_types.h:581: undefined reference to `percpu_downgrade_write'

[kbuild test robot]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git ms
head:   851deed7a22b72f2be5babed461b6cdd6c78d7b2
commit: e6b574fbfde17bd8fceb374bcb3be55c3ee23d2e [7/8] mmap_sem: Use percpu_rw_sema
config: s390-default_defconfig (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
        git checkout e6b574fbfde17bd8fceb374bcb3be55c3ee23d2e
        # save the attached .config to linux build tree
        make.cross ARCH=s390 

All errors (new ones prefixed by >>):

   fs/built-in.o: In function `downgrade_write_mmap_sem':
>> include/linux/mm_types.h:581: undefined reference to `percpu_downgrade_write'
   fs/built-in.o: In function `xfs_file_iomap_end':
   fs/xfs/xfs_iomap.c:1152: undefined reference to `put_dax'
   fs/built-in.o: In function `xfs_file_iomap_begin':
   fs/xfs/xfs_iomap.c:1071: undefined reference to `dax_get_by_host'

vim +581 include/linux/mm_types.h

   575	{
   576		lockdep_assert_held_exclusive(&mm->mmap_sem.rw_sem);
   577	}
   578	
   579	static inline void downgrade_write_mmap_sem(struct mm_struct *mm)
   580	{
 > 581		percpu_downgrade_write(&mm->mmap_sem);
   582	}
   583	
   584	static inline void mm_init_cpumask(struct mm_struct *mm)

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