cgroup: add cgroup_subsys_state->parent
With the planned unified hierarchy, css's (cgroup_subsys_state) will
be RCU protected and allowed to be attached and detached dynamically
over the course of a cgroup's lifetime. This means that css's will
stay accessible after being detached from its cgroup - the matching
pointer in cgroup->subsys[] cleared - for ref draining and RCU grace
period.
cgroup core still wants to guarantee that the parent css is never
destroyed before its children and css_parent() always returns the
parent regardless of the state of the child css as long as it's
accessible.
This patch makes css's hold onto their parents and adds css->parent so
that the parent css is never detroyed before its children and can be
determined without consulting the cgroups.
cgroup->dummy_css is also updated to point to the parent dummy_css;
however, it doesn't need to worry about object lifetime as the parent
cgroup is already pinned by the child.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 12d66fe..8a5dc91 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -75,6 +75,9 @@
/* reference count - access via css_[try]get() and css_put() */
struct percpu_ref refcnt;
+ /* the parent css */
+ struct cgroup_subsys_state *parent;
+
unsigned long flags;
/* ID for this css, if possible */
struct css_id __rcu *id;
@@ -666,15 +669,7 @@
static inline
struct cgroup_subsys_state *css_parent(struct cgroup_subsys_state *css)
{
- struct cgroup *parent_cgrp = css->cgroup->parent;
-
- if (!parent_cgrp)
- return NULL;
-
- if (css->ss)
- return parent_cgrp->subsys[css->ss->subsys_id];
- else
- return &parent_cgrp->dummy_css;
+ return css->parent;
}
/**