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

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Message-ID: <20100902171646.GC2349@linux.vnet.ibm.com>
Date:	Thu, 2 Sep 2010 10:16:46 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Miller <davem@...emloft.net>
Cc:	peterz@...radead.org, lizf@...fujitsu.com,
	herbert@...dor.hengli.com.au, davej@...hat.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cls_cgroup: Fix rcu lockdep warning

On Thu, Sep 02, 2010 at 10:05:51AM -0700, David Miller wrote:
> 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?

The rcu_dereference() is there, just buried in task_subsys_state_check()
which is called from task_subsys_state().  The code extracts the classid,
which is an integer, so the RCU-protected pointer does not leak out of
the RCU read-side critical section.

So this is OK, but the changelog should indeed say why it is OK.

							Thanx, Paul
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