From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Subject: Re: [PATCH v2 03/10] cgroup: net_cls: Protect access to
 task_cls_classid() when built as module
Date: Fri, 24 Aug 2012 16:26:21 -0700
Message-ID: <20120824232621.GQ21325@google.com>
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To: Daniel Wagner <wagi-kQCPcA+X3s7YtjvyW6yDsg@public.gmane.org>
Cc: netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Daniel Wagner <daniel.wagner-98C5kh4wR6ohFhg+JK9F0w@public.gmane.org>, "David S. Miller" <davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>, Gao feng <gaofeng-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>, Jamal Hadi Salim <jhs-jkUAjuhPggJWk0Htik3J/w@public.gmane.org>, John Fastabend <john.r.fastabend-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>, Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>, Neil Horman <nhorman-2XuSBdqkA4R54TAoqtyWWQ@public.gmane.org>

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