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Re: When are net_device names changed/changeable?

To: Andi Kleen <ak@xxxxxx>
Subject: Re: When are net_device names changed/changeable?
From: Ben Greear <greearb@xxxxxxxxxxxxxxx>
Date: Fri, 13 Oct 2000 09:14:53 -0700
Cc: linux-net <linux-net@xxxxxxxxxxxxxxxx>, "netdev@xxxxxxxxxxx" <netdev@xxxxxxxxxxx>
Organization: Candela Technologies
References: <39E419A1.DAE3F651@xxxxxxxxxxxxxxx> <20001012031620.A13200@xxxxxxxxxxx> <39E694A7.FA4AB35C@xxxxxxxxxxxxxxx> <20001013090726.44839@xxxxxxxxxxxx>
Sender: owner-netdev@xxxxxxxxxxx
Andi Kleen wrote:
> On Fri, Oct 13, 2000 at 06:00:15AM +0200, Ben Greear wrote:
> > Andi Kleen wrote:
> > >
> > > On Wed, Oct 11, 2000 at 08:54:21AM +0200, Ben Greear wrote:
> > > > I'm trying to write a hash structure that will hold the devices,
> > > > hashed by both name and index.
> > > >
> > > > It seems that the devices are first named generically: eth%d,
> > > > and at some later date, are initialized correctly.  This, of course,
> > > > completely screws up any name-based hashing I might have had...
> > > >
> > > > If names can change at any time, then I'll just quit trying to
> > > > hash on them.  However, if they only change up to some certain
> > > > step, then I'll be able to just initialize the hash list after that.
> > > >
> > > > Are the device names (for the devices in dev_base) un-changable after
> > > > some point?  What is this point?  (Would 'at-the-first-dev-add' be ok?)
> > >
> > > Root can change them anytime using SIOCSIFNAME. I wrote a small program
> > > to rename net devices based on the MAC address, which used it successfully
> > > (so eth0 was always the same adapter, no matter how the modules were 
> > > loaded)
> >
> > What if I remove the device from the hash before the name change, and
> > add it back again after?  That should preserve the hash, and I assume that
> > the devices will not be renamed regularly, so the slight performance hit
> > should not be a problem.  I'll probably have to lock the structures
> > (read/write dev_base), just as the register_netdevice() does...
> I am not sure I understand your problem. You probably want two hash
> tables, one for the ifindex and one for the ifname. These are separated.
> When ifindex or name change you update the appropiate hash table chains under
> their lock.

I have two tables, but was told that ifindex cannot change, so don't have to
worry about that one...

I didn't want to add an extra lock because I don't expect ppl to be re-naming
devices too often, and right now, the code that needs locking is already
protected by the already-existing dev_base locks.  I figure that having one
slightly coarser lock around rarely used code (change_name(..)) was preferable 
to adding another
lock around more often-run code (the dev_get_**).  I'll separate out the patch
from my VLAN code and post it for examinations this weekend..when it's written 

> > I mean, doesn't everyone need 4k VLAN interfaces?? :)
> Apparently people do, I recently had to fix a O(n^2) algorithm in ifconfig
> which made printout of thousands of interfaces very slow ...

I noticed that my hack brought an ifconfig -a of 1000 devices from about 7 
to about 1 second on my Celeron 500.  However, I think I might still be using 
an n^2
algorithm ifconfig, because it became about 20 seconds on 4k devices (with my 

> -Andi

Ben Greear (greearb@xxxxxxxxxxxxxxx)
Author of ScryMUD: 4444        (Released under GPL)     

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