netdev - Re: [PATCH net-next 2/8] rtnetlink: add rtnl_register

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Date:   Mon, 6 Nov 2017 13:44:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Florian Westphal <fw@...len.de>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net-next 2/8] rtnetlink: add rtnl_register_module

On Mon, Nov 06, 2017 at 11:51:07AM +0100, Florian Westphal wrote:
> @@ -180,6 +164,12 @@ int __rtnl_register(int protocol, int msgtype,
>  		rcu_assign_pointer(rtnl_msg_handlers[protocol], tab);
>  	}
>  
> +	WARN_ON(tab[msgindex].owner && tab[msgindex].owner != owner);
> +
> +	tab[msgindex].owner = owner;
> +	/* make sure owner is always visible first */
> +	smp_wmb();
> +
>  	if (doit)
>  		tab[msgindex].doit = doit;
>  	if (dumpit)

> @@ -235,6 +279,9 @@ int rtnl_unregister(int protocol, int msgtype)
>  	handlers[msgindex].doit = NULL;
>  	handlers[msgindex].dumpit = NULL;
>  	handlers[msgindex].flags = 0;
> +	/* make sure we clear owner last */
> +	smp_wmb();
> +	handlers[msgindex].owner = NULL;
>  	rtnl_unlock();
>  
>  	return 0;

These wmb()'s don't make sense; and the comments are incomplete. What do
they pair with? Who cares about this ordering?