[vfs:work.dcache 16/23] fs/d_path.c:122:3: error: implicit declaration of function 'prefetch'

[kbuild test robot <fengguang.wu@xxxxxxxxx>]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git work.dcache
head:   f700b37a0dc21ad5e0b5e8e947659f230fd4984c
commit: b609ffae6e69afeb1955526ff06c57795964b362 [16/23] split d_path() and friends into a separate file
config: score-spct6600_defconfig (attached as .config)
compiler: score-elf-gcc (GCC) 4.9.4
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        git checkout b609ffae6e69afeb1955526ff06c57795964b362
        # save the attached .config to linux build tree
        make.cross ARCH=score 

All errors (new ones prefixed by >>):

   fs/d_path.c: In function 'prepend_path':
>> fs/d_path.c:122:3: error: implicit declaration of function 'prefetch' [-Werror=implicit-function-declaration]
      prefetch(parent);
      ^
   cc1: some warnings being treated as errors

vim +/prefetch +122 fs/d_path.c

    55	
    56	/**
    57	 * prepend_path - Prepend path string to a buffer
    58	 * @path: the dentry/vfsmount to report
    59	 * @root: root vfsmnt/dentry
    60	 * @buffer: pointer to the end of the buffer
    61	 * @buflen: pointer to buffer length
    62	 *
    63	 * The function will first try to write out the pathname without taking any
    64	 * lock other than the RCU read lock to make sure that dentries won't go away.
    65	 * It only checks the sequence number of the global rename_lock as any change
    66	 * in the dentry's d_seq will be preceded by changes in the rename_lock
    67	 * sequence number. If the sequence number had been changed, it will restart
    68	 * the whole pathname back-tracing sequence again by taking the rename_lock.
    69	 * In this case, there is no need to take the RCU read lock as the recursive
    70	 * parent pointer references will keep the dentry chain alive as long as no
    71	 * rename operation is performed.
    72	 */
    73	static int prepend_path(const struct path *path,
    74				const struct path *root,
    75				char **buffer, int *buflen)
    76	{
    77		struct dentry *dentry;
    78		struct vfsmount *vfsmnt;
    79		struct mount *mnt;
    80		int error = 0;
    81		unsigned seq, m_seq = 0;
    82		char *bptr;
    83		int blen;
    84	
    85		rcu_read_lock();
    86	restart_mnt:
    87		read_seqbegin_or_lock(&mount_lock, &m_seq);
    88		seq = 0;
    89		rcu_read_lock();
    90	restart:
    91		bptr = *buffer;
    92		blen = *buflen;
    93		error = 0;
    94		dentry = path->dentry;
    95		vfsmnt = path->mnt;
    96		mnt = real_mount(vfsmnt);
    97		read_seqbegin_or_lock(&rename_lock, &seq);
    98		while (dentry != root->dentry || vfsmnt != root->mnt) {
    99			struct dentry * parent;
   100	
   101			if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
   102				struct mount *parent = READ_ONCE(mnt->mnt_parent);
   103				/* Escaped? */
   104				if (dentry != vfsmnt->mnt_root) {
   105					bptr = *buffer;
   106					blen = *buflen;
   107					error = 3;
   108					break;
   109				}
   110				/* Global root? */
   111				if (mnt != parent) {
   112					dentry = READ_ONCE(mnt->mnt_mountpoint);
   113					mnt = parent;
   114					vfsmnt = &mnt->mnt;
   115					continue;
   116				}
   117				if (!error)
   118					error = is_mounted(vfsmnt) ? 1 : 2;
   119				break;
   120			}
   121			parent = dentry->d_parent;
 > 122			prefetch(parent);
   123			error = prepend_name(&bptr, &blen, &dentry->d_name);
   124			if (error)
   125				break;
   126	
   127			dentry = parent;
   128		}
   129		if (!(seq & 1))
   130			rcu_read_unlock();
   131		if (need_seqretry(&rename_lock, seq)) {
   132			seq = 1;
   133			goto restart;
   134		}
   135		done_seqretry(&rename_lock, seq);
   136	
   137		if (!(m_seq & 1))
   138			rcu_read_unlock();
   139		if (need_seqretry(&mount_lock, m_seq)) {
   140			m_seq = 1;
   141			goto restart_mnt;
   142		}
   143		done_seqretry(&mount_lock, m_seq);
   144	
   145		if (error >= 0 && bptr == *buffer) {
   146			if (--blen < 0)
   147				error = -ENAMETOOLONG;
   148			else
   149				*--bptr = '/';
   150		}
   151		*buffer = bptr;
   152		*buflen = blen;
   153		return error;
   154	}
   155	

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