On Sat 03-11-12 01:38:27, Tejun Heo wrote:
> 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().
Hmm, I had to look at "cgroup_freezer: implement proper hierarchy
support" to actually understand what is the callback good for. The above
sounds as if the callback is needed when a controller wants to use
the new iterators or when pre_destroy is defined.
I think it would be helpful if the changelog described that the callback
is needed when the controller keeps a mutable shared state for the
hierarchy. For example memory controller doesn't have any such a strict
requirement so we can safely use your new iterators without pre_destroy.
Anyway, I like this change because the shared state is now really easy
to implement.
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Glauber Costa <glommer@xxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxx>
> ---
> include/linux/cgroup.h | 1 +
> kernel/cgroup.c | 12 ++++++++++--
> 2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index fe876a7..b442122 100644
> --- a/include/linux/cgroup.h
> +++ b/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);
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index e3045ad..f05d992 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -4060,10 +4060,15 @@ 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));
>
> @@ -4281,6 +4286,9 @@ 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);
> --
> 1.7.11.7
>
--
Michal Hocko
SUSE Labs
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