From mboxrd@z Thu Jan  1 00:00:00 1970
From: Daniel Wagner <wagi@monom.org>
Subject: Re: [PATCH v2 03/10] cgroup: net_cls: Protect access to task_cls_classid()
 when built as module
Date: Sat, 25 Aug 2012 18:56:29 +0200
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To: Tejun Heo <tj@kernel.org>
Cc: netdev@vger.kernel.org, cgroups@vger.kernel.org, "David S. Miller" <davem@davemloft.net>, Gao feng <gaofeng@cn.fujitsu.com>, Jamal Hadi Salim <jhs@mojatatu.com>, John Fastabend <john.r.fastabend@intel.com>, Li Zefan <lizefan@huawei.com>, Neil Horman <nhorman@tuxdriver.com>

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.

> 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.