cgroup: add cgroup_subsys->post_create()

Currently, there's no way for a controller to find out whether a new
cgroup finished all ->create() allocatinos successfully and is
considered "live" by cgroup.

This becomes a problem later when we add generic descendants walking
to cgroup which can be used by controllers as controllers don't have a
synchronization point where it can synchronize against new cgroups
appearing in such walks.

This patch adds ->post_create().  It's called after all ->create()
succeeded and the cgroup is linked into the generic cgroup hierarchy.
This plays the counterpart of ->pre_destroy().

When used in combination with the to-be-added generic descendant
iterators, ->post_create() can be used to implement reliable state
inheritance.  It will be explained with the descendant iterators.

v2: Added a paragraph about its future use w/ descendant iterators per
    Michal.

v3: Forgot to add ->post_create() invocation to cgroup_load_subsys().
    Fixed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Glauber Costa <glommer@parallels.com>

include/linux/cgroup.h

@@ -438,6 +438,7 @@ int cgroup_taskset_size(struct cgroup_taskset *tset);
 
 struct cgroup_subsys {
 	struct cgroup_subsys_state *(*create)(struct cgroup *cgrp);
+	void (*post_create)(struct cgroup *cgrp);
 	void (*pre_destroy)(struct cgroup *cgrp);
 	void (*destroy)(struct cgroup *cgrp);
 	int (*can_attach)(struct cgroup *cgrp, struct cgroup_taskset *tset);

kernel/cgroup.c

@@ -4059,10 +4059,15 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 	if (err < 0)
 		goto err_remove;
 
-	/* each css holds a ref to the cgroup's dentry */
-	for_each_subsys(root, ss)
+	for_each_subsys(root, ss) {
+		/* each css holds a ref to the cgroup's dentry */
 		dget(dentry);
 
+		/* creation succeeded, notify subsystems */
+		if (ss->post_create)
+			ss->post_create(cgrp);
+	}
+
 	/* The cgroup directory was pre-locked for us */
 	BUG_ON(!mutex_is_locked(&cgrp->dentry->d_inode->i_mutex));
 
@@ -4280,6 +4285,9 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
 
 	ss->active = 1;
 
+	if (ss->post_create)
+		ss->post_create(&ss->root->top_cgroup);
+
 	/* this function shouldn't be used with modular subsystems, since they
 	 * need to register a subsys_id, among other things */
 	BUG_ON(ss->module);
@@ -4389,6 +4397,9 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
 
 	ss->active = 1;
 
+	if (ss->post_create)
+		ss->post_create(&ss->root->top_cgroup);
+
 	/* success! */
 	mutex_unlock(&cgroup_mutex);
 	return 0;