linux-kernel - Re: [PATCH 2/2] blk-throttle: Take blkcg->lock while traversing blkcg->policy

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Message-ID: <20111020212021.GA2841@redhat.com>
Date:	Thu, 20 Oct 2011 17:20:21 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org, jaxboe@...ionio.com
Subject: Re: [PATCH 2/2] blk-throttle: Take blkcg->lock while traversing
 blkcg->policy_list

On Thu, Oct 20, 2011 at 02:11:40PM -0700, Tejun Heo wrote:
> Hello,
> 
> On Thu, Oct 20, 2011 at 05:08:26PM -0400, Vivek Goyal wrote:
> > blkcg->policy_list is protected by blkcg->lock. Its not rcu protected
> > list. So even for readers, they need to take blkcg->lock. There are
> > few functions which were reading the list without taking lock. Fix it.
> > 
> > Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> 
>  Acked-by: Tejun Heo <tj@...nel.org>
> 
> And yeap, some of rcu usages in cfq/iocg seem either incorrect or
> unnecessary.  Trying to clean up that now too.

Cool. I am waiting for the patches as I am curious to know what's incorrect
or unnecessary.

I noticed above two problems while trying to figure out how can we cleanup
the device rules when devices goes away. One place could be.

blk_throtl_exit() {
	blkiocg_del_blkio_group() {
		delete_any_policy_nodes_associated_with_this device;
	}
}

The only problem with this approach is that it will cleanup per device
weight rules also at elevator_exit() time which is not same as device
removal and one might device to bring CFQ back on device and we will
need the rules again.

cfq_exit_queue()
  cfq_release_cfq_groups
    cfq_blkiocg_del_blkio_group()
      blkiocg_del_blkio_group()

Thanks
Vivek
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