linux-kernel - Re: [PATCH 01/11] blkcg: let blkio_group point to blkio

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Date:	Thu, 2 Feb 2012 12:33:52 -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 01/11] blkcg: let blkio_group point to blkio_cgroup
 directly

Hello,

On Thu, Feb 02, 2012 at 03:03:18PM -0500, Vivek Goyal wrote:
> > +static void blkiocg_destroy(struct cgroup_subsys *subsys, struct cgroup *cgroup)
> > +{
> > +	struct blkio_cgroup *blkcg = cgroup_to_blkio_cgroup(cgroup);
> > +
> >  	if (blkcg != &blkio_root_cgroup)
> >  		kfree(blkcg);
> 
> What makes sure that all the blkg are gone and they have dropped their
> reference to blkcg? IIUC, pre-destroy will just make sure to decouple
> blkg from request queue as well as blkcg list and also drop joint cgroup
> and request queue reference.
> 
> But there could well be some IO queued in the group which might have
> its own reference and will be dropped later when IO completes. So at
> the time of blkiocg_destroy() it is not guranteed that there are
> no reference holders to blkcg?

Yeah, and that would wait till all css refs held by blkgs are gone,
right?

-- 
tejun
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