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>

1 files changed, 13 insertions, 2 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 998ab5957c6a..26f2edbaf4f5 100644
--- a/kernel/cgroup.c
+++ b/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;