linux-kernel - Re: [PATCH UPDATED 11/11] blkcg: unify blkg's for blkcg policies

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Date:	Fri, 3 Feb 2012 16:53:49 -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
Subject: Re: [PATCH UPDATED 11/11] blkcg: unify blkg's for blkcg policies

On Fri, Feb 03, 2012 at 01:47:19PM -0800, Tejun Heo wrote:
> On Fri, Feb 03, 2012 at 04:44:35PM -0500, Vivek Goyal wrote:
> > Ok, looks like now it is needed because blkcg lock will just gurantee that
> > blkg is around but blkg->pd[plid] can disappear if you are not holding
> > blkio_list lock (update_root_blkgs).
> > 
> > I am wondering if we should take blkcg->lock if blkg is on blkcg list and
> > is being modified in place. That way, once we are switching elevator,
> > we should be able to shoot down the policy data without taking blkio_list
> > lock.
> 
> I think it gotta become something per-queue, not global, and if we
> make it per-queue, it should be able to live inside queue_lock.

Hmm... then blkiocg_reset_stats() will run into lock ordering issue. Can't
hold queue lock inside blkcg lock. I guess you will do some kind of
locking trick again as you did for io context logic.

Thanks
Vivek
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