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

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Message-ID: <20120824232621.GQ21325@google.com>
Date:	Fri, 24 Aug 2012 16:26:21 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Daniel Wagner <wagi@...om.org>
Cc:	netdev@...r.kernel.org, cgroups@...r.kernel.org,
	Daniel Wagner <daniel.wagner@...-carit.de>,
	"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 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?  In general, please explain what
synchronization is going on when using sync constructs which aren't
obvious - e.g. memory barriers, rcu barriers.

Thanks.

-- 
tejun
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