linux-kernel - Re: [PATCH 20/24] block: RCU free request

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Message-ID: <20130102204355.GF11220@mtj.dyndns.org>
Date:	Wed, 2 Jan 2013 15:43:55 -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 20/24] block: RCU free request_queue

On Wed, Jan 02, 2013 at 01:48:15PM -0500, Vivek Goyal wrote:
> On Fri, Dec 28, 2012 at 12:35:42PM -0800, Tejun Heo wrote:
> > RCU free request_queue so that blkcg_gq->q can be dereferenced under
> > RCU lock.  This will be used to implement hierarchical stats.
> 
> Can we just take a blkg reference on ->q in blkg_alloc() and drop that
> reference in blkg_free(), instead of RCU freeing up queue.

I don't think we can invoke blk_release_queue() from RCU free contxt,
so we can't put request_queue from blkg_rcu_free().  Maybe we can go
through a work item but that's uglier.

Thanks.

-- 
tejun
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