cgroup: decouple cgroup_subsys_state destruction from cgroup destruction

Currently, css (cgroup_subsys_state) lifetime is tied to that of the
associated cgroup.  css's are created when the associated cgroup is
created and destroyed when it gets destroyed.  Also, individual css's
aren't RCU protected but the whole cgroup is.  With the planned
unified hierarchy, css's will need to be dynamically created and
destroyed within the lifetime of a cgroup.

To enable such usages, this patch decouples css destruction from
cgroup destruction - offline_css() invocation and the final css_put()
are moved from cgroup_destroy_css_killed() to css_killed_work_fn().
Now each css is individually offlined and put as its reference count
is killed instead of waiting for all css's attached to the cgroup to
finish refcnt killing and then proceeding to offlining and putting
them together.

While this changes the order of destruction operations, the changes
shouldn't be noticeable to cgroup subsystems or userland.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 80dca872f4d4..71e77e7cdb6f 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -82,7 +82,7 @@ struct cgroup_subsys_state {
 	/* ID for this css, if possible */
 	struct css_id __rcu *id;
 
-	/* Used to put @cgroup->dentry on the last css_put() */
+	/* percpu_ref killing and putting dentry on the last css_put() */
 	struct work_struct destroy_work;
 };