Re: Xtables2 Netlink spec

[Pablo Neira Ayuso <pablo@netfilter.org>]

On 26/11/10 20:55, Jan Engelhardt wrote:
>
> On Friday 2010-11-26 20:48, Jozsef Kadlecsik wrote:
>>>>
>>>> What is that initial data handling in dumps for?
>>>
>>> Making an atomic snapshot/copy of the table. A userspace client
>>> could take almost indefinitely on retrieving a table, so it is
>>> possible that something else changes tables meanwhile.
>>
>> Why don't you lock the tables during dumping? That way the tables won't
>> change, whatever long time the dump takes. Snapshotting the table looks as
>> wasting memory and time.
>
> For that to work, I would have to use a locking primitive that can be
> held across returns to userspace, which leaves semaphores as the only
> option and, ya, I didn't quite feel like using _that_.

Abusing the Netlink protocol to overcome the "supposed to be" limitation 
does not seem to me the way to go. Moreover, if we ever have more than X 
bytes rule-sets (I don't remember that limit that you have previously 
mentioned), you'll have to add some locking strategy anyway.

The locking is the way to go.

> Also sounds a
> bit like a killer if an admin cannot update a table just because he
> forgot some dumper process in the background in suspended state. :-/

He will notice that he did that because he hits EAGAIN, so he can kill 
the process in background and retry.