linux-kernel - Re: [PATCH 12/21] bridge: slight optimization of addr compare

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Date:	Mon, 23 Dec 2013 17:33:47 +0900
From:	Toshiaki Makita <toshiaki.makita1@...il.com>
To:	Ding Tianhong <dingtianhong@...wei.com>
Cc:	Stephen Hemminger <stephen@...workplumber.org>,
	"David S. Miller" <davem@...emloft.net>,
	bridge@...ts.linux-foundation.org, Netdev <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 12/21] bridge: slight optimization of addr compare

On Mon, 2013-12-23 at 13:10 +0800, Ding Tianhong wrote:
> Use the recently added and possibly more efficient
> ether_addr_equal_unaligned to instead of memcmp.
> 
> Cc: Stephen Hemminger <stephen@...workplumber.org>
> Cc: David Miller <davem@...emloft.net>
> Cc: bridge@...ts.linux-foundation.org
> Cc: netdev@...r.kernel.org
> Signed-off-by: Wang Weidong <wangweidong1@...wei.com>
> Signed-off-by: Ding Tianhong <dingtianhong@...wei.com>
> ---
>  net/bridge/br_stp_if.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/bridge/br_stp_if.c b/net/bridge/br_stp_if.c
> index 656a6f3..04217d1 100644
> --- a/net/bridge/br_stp_if.c
> +++ b/net/bridge/br_stp_if.c
> @@ -229,7 +229,7 @@ bool br_stp_recalculate_bridge_id(struct net_bridge *br)
>  
>  	list_for_each_entry(p, &br->port_list, list) {
>  		if (addr == br_mac_zero ||
> -		    memcmp(p->dev->dev_addr, addr, ETH_ALEN) < 0)
> +		    !ether_addr_equal_unaligned(p->dev->dev_addr, addr) < 0)
>  			addr = p->dev->dev_addr;
>  
>  	}

We cannot do this change.
!ether_addr_equal() isn't identical to memcmp().
memcmp() can return negative value but ether_addr_equal() cannot.
br_stp_recalculate_bridge_id() is searching the smallest address among
its ports. This change breaks it.

Thanks,
Toshiaki Makita

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