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Date: Thu, 19 Jan 2012 06:21:02 -0500
From: Vivek Goyal <vgoyal@redhat.com>
To: Tejun Heo <tj@kernel.org>
Cc: axboe@kernel.dk, ctalbott@google.com, rni@google.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 05/12] blkcg: update blkg get functions take blkio_cgroup
 as parameter
Message-ID: <20120119112102.GD2649@redhat.com>
References: <1326935490-11827-1-git-send-email-tj@kernel.org>
 <1326935490-11827-6-git-send-email-tj@kernel.org>
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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