Hi Ian,

I love your patch! Yet something to improve:

[auto build test ERROR on driver-core/driver-core-testing]
[also build test ERROR on v5.12-rc6 next-20210408]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Ian-Kent/kernfs-proposed-locking-and-concurrency-improvement/20210409-091821
base:   https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/driver-core.git c2f3f755f5c717f3621b33ef06d974b9cec4a104
config: arm-defconfig (attached as .config)
compiler: arm-linux-gnueabi-gcc (GCC) 9.3.0
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # https://github.com/0day-ci/linux/commit/a3608518cc551301c1c33dbf6976c360ea82a95c
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Ian-Kent/kernfs-proposed-locking-and-concurrency-improvement/20210409-091821
        git checkout a3608518cc551301c1c33dbf6976c360ea82a95c
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross ARCH=arm 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

Note: the linux-review/Ian-Kent/kernfs-proposed-locking-and-concurrency-improvement/20210409-091821 HEAD b2217b0abf0a4518541559173ff04138a6e4f083 builds fine.
      It only hurts bisectibility.

All errors (new ones prefixed by >>):

   fs/kernfs/dir.c: In function 'kernfs_dop_revalidate':
>> fs/kernfs/dir.c:1048:7: error: implicit declaration of function 'kernfs_active_read'; did you mean 'kernfs_active'? [-Werror=implicit-function-declaration]
    1048 |  if (!kernfs_active_read(kn))
         |       ^~~~~~~~~~~~~~~~~~
         |       kernfs_active
   cc1: some warnings being treated as errors


vim +1048 fs/kernfs/dir.c

  1032	
  1033	static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
  1034	{
  1035		struct kernfs_node *kn;
  1036	
  1037		if (flags & LOOKUP_RCU)
  1038			return -ECHILD;
  1039	
  1040		/* Always perform fresh lookup for negatives */
  1041		if (d_really_is_negative(dentry))
  1042			goto out_bad_unlocked;
  1043	
  1044		kn = kernfs_dentry_node(dentry);
  1045		mutex_lock(&kernfs_mutex);
  1046	
  1047		/* The kernfs node has been deactivated */
> 1048		if (!kernfs_active_read(kn))
  1049			goto out_bad;
  1050	
  1051		/* The kernfs node has been moved? */
  1052		if (kernfs_dentry_node(dentry->d_parent) != kn->parent)
  1053			goto out_bad;
  1054	
  1055		/* The kernfs node has been renamed */
  1056		if (strcmp(dentry->d_name.name, kn->name) != 0)
  1057			goto out_bad;
  1058	
  1059		/* The kernfs node has been moved to a different namespace */
  1060		if (kn->parent && kernfs_ns_enabled(kn->parent) &&
  1061		    kernfs_info(dentry->d_sb)->ns != kn->ns)
  1062			goto out_bad;
  1063	
  1064		mutex_unlock(&kernfs_mutex);
  1065		return 1;
  1066	out_bad:
  1067		mutex_unlock(&kernfs_mutex);
  1068	out_bad_unlocked:
  1069		return 0;
  1070	}
  1071	

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org