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Commit 0ae78e0b authored by Tejun Heo's avatar Tejun Heo
Browse files

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: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarLi Zefan <lizefan@huawei.com>
parent 35ef10da

include/linux/cgroup.h

+4 −9
Original line number Diff line number Diff line
@@ -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;

}



/**

kernel/cgroup.c

+15 −3
Original line number Diff line number Diff line
@@ -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);