(2012/10/31 13:22), Tejun Heo wrote:
> This patch makes cgroup_create() fail if @parent is marked removed.
> This is to prepare for further updates to cgroup_rmdir() path.
>
> Note that this change isn't strictly necessary. cgroup can only be
> created via mkdir and the removed marking and dentry removal happen
> without releasing cgroup_mutex, so cgroup_create() can never race with
> cgroup_rmdir(). Even after the scheduled updates to cgroup_rmdir(),
> cgroup_mkdir() and cgroup_rmdir() are synchronized by i_mutex
> rendering the added liveliness check unnecessary.
>
> Do it anyway such that locking is contained inside cgroup proper and
> we don't get nasty surprises if we ever grow another caller of
> cgroup_create().
>
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
I welcome this change.
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> ---
> kernel/cgroup.c | 16 +++++++++++++---
> 1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index a49cdbc..b3010ae 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -3906,6 +3906,18 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
> if (!cgrp)
> return -ENOMEM;
>
> + /*
> + * Only live parents can have children. Note that the liveliness
> + * check isn't strictly necessary because cgroup_mkdir() and
> + * cgroup_rmdir() are fully synchronized by i_mutex; however, do it
> + * anyway so that locking is contained inside cgroup proper and we
> + * don't get nasty surprises if we ever grow another caller.
> + */
> + if (!cgroup_lock_live_group(parent)) {
> + err = -ENODEV;
> + goto err_free;
> + }
> +
> /* Grab a reference on the superblock so the hierarchy doesn't
> * get deleted on unmount if there are child cgroups. This
> * can be done outside cgroup_mutex, since the sb can't
> @@ -3913,8 +3925,6 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
> * fs */
> atomic_inc(&sb->s_active);
>
> - mutex_lock(&cgroup_mutex);
> -
> init_cgroup_housekeeping(cgrp);
>
> cgrp->parent = parent;
> @@ -3985,7 +3995,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
>
> /* Release the reference count that we took on the superblock */
> deactivate_super(sb);
> -
> +err_free:
> kfree(cgrp);
> return err;
> }
>
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