linux-kernel - Re: [PATCH 2/9] blkcg: drop unnecessary RCU locking

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Message-ID: <20120217181634.GD29414@google.com>
Date:	Fri, 17 Feb 2012 10:16:34 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	axboe@...nel.dk, ctalbott@...gle.com, rni@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/9] blkcg: drop unnecessary RCU locking

On Fri, Feb 17, 2012 at 01:08:00PM -0500, Vivek Goyal wrote:
> > Modifying blkgs require both blkcg and queue locks,
> > so read access can be done holding any lock.
> 
> This is the point I am not getting. How blkg_lookup() is safe just
> under queue lock. What stops freeing up blkg associated with other
> queues. I thought caller needs to hold rcu_read_lock() also to
> make sure it can safely compare blkg->q == q and return the blkg
> belonging to the queue in question.

Ooh, you're right.  I got confused.  We should be holding either blkcg
lock or rcu_read_lock() across blkg_lookup().  Will update.

Thanks.

-- 
tejun
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