Re: [PATCH v2 03/10] cgroup: net_cls: Protect access to task_cls_classid() when built as module

[Daniel Wagner <wagi@monom.org>]

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.