linux-kernel - Re: [PATCH block/for-3.4/core] cfq: fix cfqg ref handling when BLK_CGROUP && !CFQ_GROUP

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Date:	Thu, 15 Mar 2012 09:50:57 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	Jens Axboe <axboe@...nel.dk>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH block/for-3.4/core] cfq: fix cfqg ref handling when
 BLK_CGROUP && !CFQ_GROUP_IOSCHED

Hello,

On Thu, Mar 15, 2012 at 12:46:21PM -0400, Vivek Goyal wrote:
> On Thu, Mar 15, 2012 at 09:35:46AM -0700, Tejun Heo wrote:
> 
> [..]
> > ---
> > Yeap, forgot to test that config combination.  Fengguang, can you
> > please test this patch?  Jens, once Fengguang confirms the fix, can
> > you please apply this on top of for-3.4/core along with the pending
> > stats updates?
> 
> I was working on similar patch and you beat me to it. :-) One comment
> below.

:)

> [..]
> > @@ -3533,7 +3551,7 @@ static int cfq_init_queue(struct request
> >  
> >  	spin_lock_irq(q->queue_lock);
> >  	cfq_link_cfqq_cfqg(&cfqd->oom_cfqq, cfqd->root_group);
> > -	blkg_put(cfqg_to_blkg(cfqd->root_group));
> > +	cfqg_put(cfqd->root_group);
> 
> This seems to be a spurious cfqg_put()? Which reference we are putting
> down here?

The extra ref from cfq_link_cfqq_cfqg() for oom_cfqq; otherwise, we
need an extra cfq_put() in cfq_exit_queue().  I thought I wrote
comment about that somewhere.  Hmmm.... apparently not.  The thing is
that oom_cfqq doesn't go through proper cfqq destruction and thus
never puts the extra ref to root cfqg.

Thanks.

-- 
tejun
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