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cgroup: add cgroup_subsys_state->parent

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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>
This commit is contained in:
Tejun Heo
2013-08-13 11:01:54 -04:00
parent 35ef10da65
commit 0ae78e0bf1
2 changed files with 19 additions and 12 deletions
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18
kernel/cgroup.c
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@ -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);