kernfs, cgroup: implement kernfs_show() and cgroup_file_show()

Hello,

Greg: If this looks good to you, can you apply 0001-0008 to your tree? I'll
likely have more cgroup updates on top, so I think it'd be better if I pull
your tree and then handle the cgroup part in cgroup tree.

Currently, deactivated kernfs nodes are used for two purposes - during
removal to kill and drain nodes and during creation to make multiple
kernfs_nodes creations to succeed or fail as a group.

This patchset implements kernfs_show() which can dynamically show and hide
kernfs_nodes using the [de]activation mechanisms, and, on top, implements
cgroup_file_show() which allows toggling cgroup file visiblity.

This is for the following pending patchset to allow disabling PSI on
per-cgroup basis:

 https://lore.kernel.org/all/20220808110341.15799-1-zhouch...

which requires hiding the corresponding cgroup interface files while
disabled.

This patchset contains the following nine patches.

 0001-kernfs-Simply-by-replacing-kernfs_deref_open_node-wi.patch
 0002-kernfs-Drop-unnecessary-mutex-local-variable-initial.patch
 0003-kernfs-Refactor-kernfs_get_open_node.patch
 0004-kernfs-Skip-kernfs_drain_open_files-more-aggressivel.patch
 0005-kernfs-Improve-kernfs_drain-and-always-call-on-remov.patch
 0006-kernfs-Add-KERNFS_REMOVING-flags.patch
 0007-kernfs-Factor-out-kernfs_activate_one.patch
 0008-kernfs-Implement-kernfs_show.patch
 0009-cgroup-Implement-cgroup_file_show.patch

0001-0003 are misc prep patches. 0004-0008 implement kernsf_deactivate().
0009 implements cgroup_file_show() on top. The patches are also available in
the following git branch:

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git kernfs-show

The main difference from the previous version[1] is that while show/hide
still use the same mechanism as [de]activation, their states are tracked
separately so that e.g. an unrelated kernfs_activate() higher up in the tree
doesn't interfere with hidden nodes in the subtree.

diffstat follows. Thanks.

 fs/kernfs/dir.c             |  102 +++++++++++++++++++++++++++++++----------------
 fs/kernfs/file.c            |  151 +++++++++++++++++++++++++++++++++-------------------------------------
 fs/kernfs/kernfs-internal.h |    1
 include/linux/cgroup.h      |    1
 include/linux/kernfs.h      |    3 +
 kernel/cgroup/cgroup.c      |   20 +++++++++
 6 files changed, 166 insertions(+), 112 deletions(-)

--
tejun

[1] http://lkml.kernel.org/r/20220820000550.367085-1-tj@kerne...