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

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Message-ID: <20120119154515.GC5198@google.com>
Date:	Thu, 19 Jan 2012 07:45:15 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
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 Thu, Jan 19, 2012 at 06:21:02AM -0500, Vivek Goyal wrote:
> 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?

Yeah, for now but later patch will make blkg's hold onto the
associated blkcg and this reference will be dropped on blkg
destruction.

Thanks.

-- 
tejun
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