Subject: Re: [PATCH 12/21] bridge: slight optimization of addr compare

From: Ding Tianhong <[email-masked]>
Date: Mon, 23 Dec 2013 16:38:01 +0800

On 2013/12/23 16:33, Toshiaki Makita wrote:
> 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 <[email-masked]>
>> Cc: David Miller <[email-masked]>
>> Cc: [email-masked]
>> Cc: [email-masked]
>> Signed-off-by: Wang Weidong <[email-masked]>
>> Signed-off-by: Ding Tianhong <[email-masked]>
>> ---
>>  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

Yes, I miss it, the negative value is useful here, thanks for point out.

Regards
Ding