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Subject[PATCH 1/9 v2] 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

Signed-off-by: Tejun Heo <>
Acked-by: Michal Hocko <>
Cc: Glauber Costa <>
If you can explain it better, please be my guest.


include/linux/cgroup.h | 1 +
kernel/cgroup.c | 12 ++++++++++--
2 files changed, 11 insertions(+), 2 deletions(-)

--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -438,6 +438,7 @@ int cgroup_taskset_size(struct cgroup_ta

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);
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4060,10 +4060,15 @@ static long cgroup_create(struct cgroup
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 */

+ /* creation succeeded, notify subsystems */
+ if (ss->post_create)
+ ss->post_create(cgrp);
+ }
/* The cgroup directory was pre-locked for us */

@@ -4281,6 +4286,9 @@ static void __init cgroup_init_subsys(st

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 */

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