linux-kernel - Re: [PATCH 21/24] blkcg: make blkcg_print_blkgs() grab q locks instead of blkcg lock

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Message-ID: <20130102204527.GG11220@mtj.dyndns.org>
Date:	Wed, 2 Jan 2013 15:45:27 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	lizefan@...wei.com, axboe@...nel.dk,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, ctalbott@...gle.com, rni@...gle.com
Subject: Re: [PATCH 21/24] blkcg: make blkcg_print_blkgs() grab q locks
 instead of blkcg lock

On Wed, Jan 02, 2013 at 02:27:00PM -0500, Vivek Goyal wrote:
> On Fri, Dec 28, 2012 at 12:35:43PM -0800, Tejun Heo wrote:
> > Instead of holding blkcg->lock while walking ->blkg_list and executing
> > prfill(), RCU walk ->blkg_list and hold the blkg's queue lock while
> > executing prfill().  This makes prfill() implementations easier as
> > stats are mostly protected by queue lock.
> > 
> > This will be used to implement hierarchical stats.
> > 
> 
> Hi Tejun,
> 
> I think dropping blkcg->lock might be a problem. Using RCU we have made
> sure that blkg and q are around. But what about blkg->q.backing_dev_info.dev.
> 
> We can follow bdi->dev pointer in blkg_dev_name(). I am not sure if we
> ever clear it from q or not when device goes away.

If the queue is dead, it wouldn't have policy enabled bit set which is
tested while holding queue lock, so I don't think it's gonna be a
problem.

Thanks.

-- 
tejun
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