linux-kernel - Re: [PATCH net-next] net: rtnetlink: convert rcu_assign_pointer to RCU_INIT

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Date:   Thu, 23 Sep 2021 10:09:25 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Yajun Deng <yajun.deng@...ux.dev>, davem@...emloft.net,
        kuba@...nel.org
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] net: rtnetlink: convert rcu_assign_pointer to
 RCU_INIT_POINTER



On 9/17/21 11:36 PM, Yajun Deng wrote:
> It no need barrier when assigning a NULL value to an RCU protected
> pointer. So use RCU_INIT_POINTER() instead for more fast.
> 
> Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>
> ---
>  net/core/rtnetlink.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
> index 972c8cb303a5..327ca6bc6e6d 100644
> --- a/net/core/rtnetlink.c
> +++ b/net/core/rtnetlink.c
> @@ -301,7 +301,7 @@ int rtnl_unregister(int protocol, int msgtype)
>  	}
>  
>  	link = rtnl_dereference(tab[msgindex]);
> -	rcu_assign_pointer(tab[msgindex], NULL);
> +	RCU_INIT_POINTER(tab[msgindex], NULL);
>  	rtnl_unlock();
>  
>  	kfree_rcu(link, rcu);
> @@ -337,7 +337,7 @@ void rtnl_unregister_all(int protocol)
>  		if (!link)
>  			continue;
>  
> -		rcu_assign_pointer(tab[msgindex], NULL);
> +		RCU_INIT_POINTER(tab[msgindex], NULL);
>  		kfree_rcu(link, rcu);
>  	}
>  	rtnl_unlock();
> 

FYI, there is no memory barrier involved in 

rcu_assign_pointer(tab[msgindex], NULL);

This has been the case for the last 5 years.

Your patch was not needed really.