From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH] cls_cgroup: Fix rcu lockdep warning Date: Thu, 2 Sep 2010 10:16:46 -0700 Message-ID: <20100902171646.GC2349@linux.vnet.ibm.com> References: <4C7F446F.8040303@cn.fujitsu.com> <1283423916.2059.1847.camel@laptop> <20100902.100551.115929058.davem@davemloft.net> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii 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 To: David Miller Return-path: Content-Disposition: inline In-Reply-To: <20100902.100551.115929058.davem@davemloft.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Thu, Sep 02, 2010 at 10:05:51AM -0700, David Miller wrote: > From: Peter Zijlstra > 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