From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH net-next 3/6] tcp_metrics: Add a field tcpm_net and
 verify it matches on lookup
Date: Tue, 10 Mar 2015 13:29:01 -0400 (EDT)
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From: ebiederm@xmission.com (Eric W. Biederman)
Date: Tue, 10 Mar 2015 12:06:32 -0500

> David Miller <davem@davemloft.net> writes:
> 
>> From: ebiederm@xmission.com (Eric W. Biederman)
>> Date: Tue, 10 Mar 2015 01:59:48 -0500
>>
>>> If we actually really care about struct net going away it would be
>>> much better to globally replace struct net with a typedef that looks
>>> something like:
>>> 
>>> #ifdef CONFIG_NET_NS
>>> struct net_ref {
>>>        struct net *net;
>>> };
>>> #else
>>> struct net_ref {
>>> };
>>> #endif
>>> typedef struct net_ref net_t;
>>> 
>>> That would remove the need for write_pnet and read_pnet, make it
>>> impossible to forget net_eq and make network namespace arguments to
>>> functions also boil away at compile time if the network namespace code
>>> was not enabled.
>>> 
>>> That was the original design and I forget why we didn't do that with
>>> struct net.  But we did not.
>>
>> This keeps the ifdefs out of foo.c code, so I like it.
> 
> Alright.  It does wind up requiring things like:

Another approach is to use a macro for the instantiation of a "struct
net *" member.

It could evaluate to "struct { } x;" when NETNS is disabled.

Then you don't need all the special accessors, read_pnet() and
write_pnet() are sufficient.