linux-kernel - Re: [PATCH] cls_cgroup: Fix rcu lockdep warning

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Message-Id: <20100902.100551.115929058.davem@davemloft.net>
Date:	Thu, 02 Sep 2010 10:05:51 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	peterz@...radead.org
Cc:	lizf@...fujitsu.com, herbert@...dor.hengli.com.au,
	davej@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] cls_cgroup: Fix rcu lockdep warning

From: Peter Zijlstra <peterz@...radead.org>
Date: Thu, 02 Sep 2010 12:38:36 +0200

> On Thu, 2010-09-02 at 14:30 +0800, Li Zefan wrote:
>> Calling task_subsys_state() without holding rcu_read_lock or
>> cgroup_mutex can cause lockdep warning.
>> 
> 
> That is not a suitable changelog.
> 
> Was the warning correct? Is your patch correct? What does RCU protect
> here and why can we use classid after dropping it.
> 
> Simply frobbing code to make the warning go away is not good.

In fact shouldn't this be a rcu_dereference() or similar?
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