From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: David Miller <davem@davemloft.net>
Cc: peterz@infradead.org, lizf@cn.fujitsu.com,
herbert@gondor.hengli.com.au, davej@redhat.com,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] cls_cgroup: Fix rcu lockdep warning
Date: Thu, 2 Sep 2010 10:16:46 -0700 [thread overview]
Message-ID: <20100902171646.GC2349@linux.vnet.ibm.com> (raw)
In-Reply-To: <20100902.100551.115929058.davem@davemloft.net>
On Thu, Sep 02, 2010 at 10:05:51AM -0700, David Miller wrote:
> From: Peter Zijlstra <peterz@infradead.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
next prev parent reply other threads:[~2010-09-02 17:16 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-09-02 6:30 [PATCH] cls_cgroup: Fix rcu lockdep warning Li Zefan
2010-09-02 10:38 ` Peter Zijlstra
2010-09-02 17:05 ` David Miller
2010-09-02 17:16 ` Paul E. McKenney [this message]
2010-09-03 1:10 ` Li Zefan
2010-09-03 4:52 ` Herbert Xu
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