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@xxxxxxxxxx>
---
include/linux/cgroup.h | 13 ++++---------
kernel/cgroup.c | 18 +++++++++++++++---
2 files changed, 19 insertions(+), 12 deletions(-)
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 @@ 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 0b28097..5c6dd7e 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);
--
1.8.3.1
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