linux-kernel - Re: [PATCH v12 7/8] cgroup: add a tset_get

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Message-ID: <20150518213423.GI24861@htj.duckdns.org>
Date:	Mon, 18 May 2015 17:34:23 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Aleksa Sarai <cyphar@...har.com>
Cc:	lizefan@...wei.com, mingo@...hat.com, peterz@...radead.org,
	richard@....at, fweisbec@...il.com, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org
Subject: Re: [PATCH v12 7/8] cgroup: add a tset_get_css macro

On Tue, May 19, 2015 at 12:51:06AM +1000, Aleksa Sarai wrote:
> This adds a macro to get the css of a tset (using task_get_css()) by
> just grabbing a ref to the first item in the tset (since there is a
> guarantee that all tasks in a tset share a css).
> 
> This is in preparation for implementing the pids cgroup subsystem.
> 
> Signed-off-by: Aleksa Sarai <cyphar@...har.com>
> ---
>  include/linux/cgroup.h | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 886a883..773846d 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -639,6 +639,18 @@ struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset);
>  	for ((task) = cgroup_taskset_first((tset)); (task);		\
>  	     (task) = cgroup_taskset_next((tset)))
>  
> +/**
> + * tset_get_css - obtain and get css for (tset, subsys_id)
> + * @tset: target taskset
> + * @subsys_id: target subsystem id
> + *
> + * Since all of the tasks in a taskset are guaranteed to have the same css, it's
> + * safe to grab the ref of just the first task's css and treat it as though you
> + * have a ref on the taskset's "collective" css.
> + */
> +#define tset_get_css(tset, subsys_id)					\
> +	task_get_css(cgroup_taskset_first(tset), subsys_id)

They aren't tho.  It depends on the hierarchy configuration.  csses
belonging to hierarchies which aren't the current migration target may
differ.

Thanks.

-- 
tejun
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