Re: [PATCH RFC/RFT net-next 00/17] net: Convert neighbor tables to per-namespace

[David Ahern <dsahern@gmail.com>]

On 7/18/18 6:54 PM, Michael Richardson wrote:
>> Remove open use of arp_tbl and nd_tbl in favor of the new
>> ipv{4,6}_neigh_table helpers. Since the existence of the IPv6 table
>> is managed by the core networking, the IS_ENABLED checks for IPv6
>> can be removed in favor of "is the table non-NULL".
> 
> What's the advantage of changing this check? (I am ignorant)
> 

Just makes the code simpler.

The current nd_tbl is a global owned by the ipv6 code
(net/ipv6/ndisc.c). If CONFIG_IPV6 is not enabled, then nd_tbl is not
defined which leads code referencing it to use if checks like this:

#if IS_ENABLED(CONFIG_IPV6)
		if (!p->dev || (p->tbl != &nd_tbl && p->tbl != &arp_tbl))
#else
		if (!p->dev || p->tbl != &arp_tbl)
#endif

With the neigh_find_table approach the IS_ENABLED can be removed in
favor of 'if (tbl)'. If tbl is set, then ipv6 is loaded and initialized.