linux-kernel - Re: [PATCH 05/12] blkcg: update blkg get functions take blkio

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Message-ID: <20120119112102.GD2649@redhat.com>
Date:	Thu, 19 Jan 2012 06:21:02 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	axboe@...nel.dk, ctalbott@...gle.com, rni@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/12] blkcg: update blkg get functions take blkio_cgroup
 as parameter

On Wed, Jan 18, 2012 at 05:11:23PM -0800, Tejun Heo wrote:

[..]
> -static struct throtl_grp * throtl_get_tg(struct throtl_data *td)
> +static struct throtl_grp *throtl_get_tg(struct throtl_data *td,
> +					struct blkio_cgroup *blkcg)
>  {
>  	struct throtl_grp *tg = NULL, *__tg = NULL;
> -	struct blkio_cgroup *blkcg;
>  	struct request_queue *q = td->queue;
>  
>  	/* no throttling for dead queue */
>  	if (unlikely(blk_queue_dead(q)))
>  		return NULL;
>  
> -	blkcg = task_blkio_cgroup(current);
>  	tg = throtl_find_tg(td, blkcg);
>  	if (tg)
>  		return tg;
>  
> +	if (!css_tryget(&blkcg->css))
> +		return NULL;
> +

This reference is to make sure that blkcg does not go away while we drop
the rcu lock and do the group allocation?

Thanks
Vivek
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