linux-kernel - Re: [PATCH 14/16] blkcg: don't allow or retain configuration of missing devices

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Message-ID: <20120124163224.GB17291@redhat.com>
Date:	Tue, 24 Jan 2012 11:32:24 -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, Kay Sievers <kay.sievers@...y.org>
Subject: Re: [PATCH 14/16] blkcg: don't allow or retain configuration of
 missing devices

On Tue, Jan 24, 2012 at 07:53:09AM -0800, Tejun Heo wrote:
> On Tue, Jan 24, 2012 at 10:42:24AM -0500, Vivek Goyal wrote:
> > On Mon, Jan 23, 2012 at 03:09:51PM -0800, Tejun Heo wrote:
> > > +	disk = get_gendisk(dev, &part);
> > > +
> > > +	if ((!disk || part) && temp) {
> > > +		ret = -ENODEV;
> > > +		goto out;
> > > +	}
> > > +
> > > +	rcu_read_lock();
> > > +
> > > +	if (disk && !part) {
> > > +		spin_lock_irq(disk->queue->queue_lock);
> > > +		blkg = blkg_lookup_create(blkcg, disk->queue, plid);
> > > +		spin_unlock_irq(disk->queue->queue_lock);
> > 
> > If queue is in bypass mode, is group creation and linking allowed when
> > somebody is trying to set per device rules. (/me is thinking if there are
> > any potential races between elevator switch and rule setting in cgroup).
> 
> Yeap, there is.  Nice catch.  blkg_lookup_create() should be testing
> blk_queue_bypass() instead of blk_queue_dead() and parse_and_set
> should probably retry after a bit.  Will update.

So now group creation fails if queue is in bypass mode. So a per device
rule setting will fail both for cfq and throttling logic while device
is still around. So a user is supposed to retry? How would a user know 
that elevator switch is happening and he should retry and till when should
he retry.

Thanks
Vivek
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