[3/8] cgroup: implement cgroup_get_from_path() and expose cgroup_put()

Implement cgroup_get_from_path() using kernfs_walk_and_get() which
obtains a default hierarchy cgroup from its path.  This will be used
to allow cgroup path based matching from outside cgroup proper -
e.g. networking and perf.

v2: Add EXPORT_SYMBOL_GPL(cgroup_get_from_path).

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 include/linux/cgroup.h |  7 +++++++
 kernel/cgroup.c        | 39 ++++++++++++++++++++++++++++++++++-----
 2 files changed, 41 insertions(+), 5 deletions(-)

On Mon, Dec 07, 2015 at 05:38:50PM -0500, Tejun Heo wrote:
> Implement cgroup_get_from_path() using kernfs_walk_and_get() which
> obtains a default hierarchy cgroup from its path.  This will be used
> to allow cgroup path based matching from outside cgroup proper -
> e.g. networking and perf.

Hi Tejun,

I'm trying to figure out how to handle this in the cgroup ns patchset.
Is this going to be purely used internally?  From the user i see in
this patchset it looks like I should leave it be (and have @path always
be absolute)  Is that right?

thanks,
-serge

> v2: Add EXPORT_SYMBOL_GPL(cgroup_get_from_path).
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
>  include/linux/cgroup.h |  7 +++++++
>  kernel/cgroup.c        | 39 ++++++++++++++++++++++++++++++++++-----
>  2 files changed, 41 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index b5ee2c4..4c3ffab 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -81,6 +81,8 @@ struct cgroup_subsys_state *cgroup_get_e_css(struct cgroup *cgroup,
>  struct cgroup_subsys_state *css_tryget_online_from_dir(struct dentry *dentry,
>  						       struct cgroup_subsys *ss);
>  
> +struct cgroup *cgroup_get_from_path(const char *path);
> +
>  int cgroup_attach_task_all(struct task_struct *from, struct task_struct *);
>  int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from);
>  
> @@ -351,6 +353,11 @@ static inline void css_put_many(struct cgroup_subsys_state *css, unsigned int n)
>  		percpu_ref_put_many(&css->refcnt, n);
>  }
>  
> +static inline void cgroup_put(struct cgroup *cgrp)
> +{
> +	css_put(&cgrp->self);
> +}
> +
>  /**
>   * task_css_set_check - obtain a task's css_set with extra access conditions
>   * @task: the task to obtain css_set for
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 3190040..3db5e8f 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -434,11 +434,6 @@ static bool cgroup_tryget(struct cgroup *cgrp)
>  	return css_tryget(&cgrp->self);
>  }
>  
> -static void cgroup_put(struct cgroup *cgrp)
> -{
> -	css_put(&cgrp->self);
> -}
> -
>  struct cgroup_subsys_state *of_css(struct kernfs_open_file *of)
>  {
>  	struct cgroup *cgrp = of->kn->parent->priv;
> @@ -5753,6 +5748,40 @@ struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)
>  	return id > 0 ? idr_find(&ss->css_idr, id) : NULL;
>  }
>  
> +/**
> + * cgroup_get_from_path - lookup and get a cgroup from its default hierarchy path
> + * @path: path on the default hierarchy
> + *
> + * Find the cgroup at @path on the default hierarchy, increment its
> + * reference count and return it.  Returns pointer to the found cgroup on
> + * success, ERR_PTR(-ENOENT) if @path doens't exist and ERR_PTR(-ENOTDIR)
> + * if @path points to a non-directory.
> + */
> +struct cgroup *cgroup_get_from_path(const char *path)
> +{
> +	struct kernfs_node *kn;
> +	struct cgroup *cgrp;
> +
> +	mutex_lock(&cgroup_mutex);
> +
> +	kn = kernfs_walk_and_get(cgrp_dfl_root.cgrp.kn, path);
> +	if (kn) {
> +		if (kernfs_type(kn) == KERNFS_DIR) {
> +			cgrp = kn->priv;
> +			cgroup_get(cgrp);
> +		} else {
> +			cgrp = ERR_PTR(-ENOTDIR);
> +		}
> +		kernfs_put(kn);
> +	} else {
> +		cgrp = ERR_PTR(-ENOENT);
> +	}
> +
> +	mutex_unlock(&cgroup_mutex);
> +	return cgrp;
> +}
> +EXPORT_SYMBOL_GPL(cgroup_get_from_path);
> +
>  #ifdef CONFIG_CGROUP_DEBUG
>  static struct cgroup_subsys_state *
>  debug_css_alloc(struct cgroup_subsys_state *parent_css)
> -- 
> 2.5.0
> 
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Hello, Serge.

On Mon, Dec 21, 2015 at 06:22:41PM -0600, Serge E. Hallyn wrote:
> I'm trying to figure out how to handle this in the cgroup ns patchset.
> Is this going to be purely used internally?  From the user i see in
> this patchset it looks like I should leave it be (and have @path always
> be absolute)  Is that right?

Yeap, I don't think this function needs to worry about namespaces.

Thanks.