linux-kernel - Re: [PATCH 3/8] cgroup: add function to get task's cgroup

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Message-ID: <20151124162728.GN17033@mtj.duckdns.org>
Date:	Tue, 24 Nov 2015 11:27:28 -0500
From:	Tejun Heo <tj@...nel.org>
To:	serge@...lyn.com
Cc:	linux-kernel@...r.kernel.org, adityakali@...gle.com,
	linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
	cgroups@...r.kernel.org, lxc-devel@...ts.linuxcontainers.org,
	akpm@...ux-foundation.org, ebiederm@...ssion.com,
	Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH 3/8] cgroup: add function to get task's cgroup

Hello,

On Mon, Nov 16, 2015 at 01:51:40PM -0600, serge@...lyn.com wrote:
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 22e3754..29f0b02 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -326,6 +326,7 @@ static inline bool css_tryget_online(struct cgroup_subsys_state *css)
>  		return percpu_ref_tryget_live(&css->refcnt);
>  	return true;
>  }
> +struct cgroup *get_task_cgroup(struct task_struct *task);

Please move this where other prototypes are.

> +/*
> + * get_task_cgroup - returns the cgroup of the task in the default cgroup
> + * hierarchy.
> + *
> + * @task: target task
> + * This function returns the @task's cgroup on the default cgroup hierarchy. The
> + * returned cgroup has its reference incremented (by calling cgroup_get()). So
> + * the caller must cgroup_put() the obtained reference once it is done with it.
> + */
> +struct cgroup *get_task_cgroup(struct task_struct *task)
> +{
> +	struct cgroup *cgrp;
> +
> +	mutex_lock(&cgroup_mutex);
> +	spin_lock_bh(&css_set_lock);
> +
> +	cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
> +	cgroup_get(cgrp);
> +
> +	spin_unlock_bh(&css_set_lock);
> +	mutex_unlock(&cgroup_mutex);
> +	return cgrp;
> +}
> +EXPORT_SYMBOL_GPL(get_task_cgroup);

So, exposing cgroup_mutex this way can lead to ugly lock dependency
issues as cgroup_mutex is expected to be outside of pretty much
everything.  task_cgroup_path() does it but it has no users (should
prolly removed) and cgroup_attach_task_all() is pretty specific.

Hmmm... cc'ing Li (btw, please cc him and Johannes from the next
posting).  Li, I don't think cset_cgroup_from_root() really needs
cgroup_mutex.  css_set_lock seems to be enough.  What do you think?

Thanks.

-- 
tejun
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