netdev - Re: [PATCH v2 03/10] cgroup: net_cls: Protect access to task_cls

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Date:	Tue, 28 Aug 2012 07:47:25 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Daniel Wagner <wagi@...om.org>
Cc:	Tejun Heo <tj@...nel.org>, netdev@...r.kernel.org,
	cgroups@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Gao feng <gaofeng@...fujitsu.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Li Zefan <lizefan@...wei.com>,
	Neil Horman <nhorman@...driver.com>
Subject: Re: [PATCH v2 03/10] cgroup: net_cls: Protect access to
 task_cls_classid() when built as module

On Sat, Aug 25, 2012 at 06:56:29PM +0200, Daniel Wagner wrote:
> On 25.08.2012 01:26, Tejun Heo wrote:
> >On Fri, Aug 24, 2012 at 04:01:37PM +0200, Daniel Wagner wrote:
> >>@@ -306,6 +312,11 @@ static void __exit exit_cgroup_cls(void)
> >>  	synchronize_rcu();
> >>  #endif
> >>
> >>+#if IS_MODULE(CONFIG_NET_CLS_CGROUP)
> >>+	static_key_slow_dec(&cgroup_cls_enabled);
> >>+	rcu_barrier();
> >
> >Why is this rcu_barrier() necessary?
> 
> I have read the rcubarrier.txt document and I got from that that an
> rcu_barrier() is needed when unloading a module. But maybe I got it
> wrong.
> 
> So the idea after disabling the jump lables all pending readers in
> task_cls_classid() have left. THe same thing is done in the old code
> with the dynamic id part. With the difference that synchronize_rcu()
> is used.

FWIW, the rcu_barrier() is needed only if the module uses call_rcu().
In that case it is required to ensure that all the resulting callbacks
execute before the module's .text is freed up.

							Thanx, Paul

> >In general, please explain what
> >synchronization is going on when using sync constructs which aren't
> >obvious - e.g. memory barriers, rcu barriers.
> 
> Sure, I will keep this in mind.
> 
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