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>

2 files changed, 19 insertions, 12 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 12d66fee26f8..8a5dc91fbaad 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -75,6 +75,9 @@ struct cgroup_subsys_state {
         /* 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 @@ struct cgroup_subsys {
 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;
 }
 
 /**
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 0b280978f097..5c6dd7ed26a7 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4264,6 +4264,9 @@ static void css_free_work_fn(struct work_struct *work)
         struct cgroup_subsys_state *css =
                 container_of(work, struct cgroup_subsys_state, destroy_work);
 
+        if (css->parent)
+                css_put(css->parent);
+
         cgroup_dput(css->cgroup);
 }
 
@@ -4290,8 +4293,12 @@ static void init_cgroup_css(struct cgroup_subsys_state *css,
         css->ss = ss;
         css->flags = 0;
         css->id = NULL;
-        if (cgrp == cgroup_dummy_top)
+
+        if (cgrp->parent)
+                css->parent = cgroup_css(cgrp->parent, ss->subsys_id);
+        else
                 css->flags |= CSS_ROOT;
+
         BUG_ON(cgroup_css(cgrp, ss->subsys_id));
         cgrp->subsys[ss->subsys_id] = css;
 }
@@ -4388,6 +4395,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
         cgrp->dentry = dentry;
 
         cgrp->parent = parent;
+        cgrp->dummy_css.parent = &parent->dummy_css;
         cgrp->root = parent->root;
 
         if (notify_on_release(parent))
@@ -4436,9 +4444,13 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
         list_add_tail_rcu(&cgrp->sibling, &cgrp->parent->children);
         root->number_of_cgroups++;
 
-        /* each css holds a ref to the cgroup's dentry */
-        for_each_root_subsys(root, ss)
+        /* each css holds a ref to the cgroup's dentry and the parent css */
+        for_each_root_subsys(root, ss) {
+                struct cgroup_subsys_state *css = cgroup_css(cgrp, ss->subsys_id);
+
                 dget(dentry);
+                percpu_ref_get(&css->parent->refcnt);
+        }
 
         /* hold a ref to the parent's dentry */
         dget(parent->dentry);