linux-kernel - Re: [PATCH 01/12] blkcg: obtaining blkg should be enclosed inside rcu_read

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Date:	Thu, 19 Jan 2012 07:58:44 -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 01/12] blkcg: obtaining blkg should be enclosed inside
 rcu_read_lock()

On Thu, Jan 19, 2012 at 10:54:45AM -0500, Vivek Goyal wrote:
> > No, no matter whatever synchronization scheme is in use, the code is
> > seriously screwed up if it's doing something like,
> > 
> > 	lock();
> > 	a = lookup();
> > 	unlock();
> > 	return a;
> > 
> > You should *NEVER* be doing that.
> 
> I guess ioc_lookup_icq() is doing something similar. We call it under
> queue lock. Take rcu lock inside for sanity of radix tree and then 
> release rcu lock and return icq.

Yeap, it is.  We should be using rcu_dereference_check() there too.

Thanks.

-- 
tejun
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