netdev - Re: [PATCH net-2.6.26 2/2][NETNS]: The generic per-net pointers.

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Date:	Tue, 15 Apr 2008 00:36:23 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	paulmck@...ux.vnet.ibm.com
Cc:	xemul@...nvz.org, netdev@...r.kernel.org, dlezcano@...ibm.com
Subject: Re: [PATCH net-2.6.26 2/2][NETNS]: The generic per-net pointers.

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Date: Mon, 14 Apr 2008 09:22:20 -0700

> On Sat, Apr 12, 2008 at 01:44:45PM +0400, Pavel Emelyanov wrote:
> > Add the elastic array of void * pointer to the struct net.
> > The access rules are simple:
> > 
> >  1. register the ops with register_pernet_gen_device to get
> >     the id of your private pointer
> >  2. call net_assign_generic() to put the private data on the
> >     struct net (most preferably this should be done in the
> >     ->init callback of the ops registered)
> >  3. do not store any private reference on the net_generic array;
> >  4. do not change this pointer while the net is alive;
> >  5. use the net_generic() to get the pointer.
> > 
> > When adding a new pointer, I copy the old array, replace it
> > with a new one and schedule the old for kfree after an RCU
> > grace period.
> > 
> > Since the net_generic explores the net->gen array inside rcu
> > read section and once set the net->gen->ptr[x] pointer never 
> > changes, this grants us a safe access to generic pointers.
> > 
> > Quoting Paul: "... RCU is protecting -only- the net_generic 
> > structure that net_generic() is traversing, and the [pointer]
> > returned by net_generic() is protected by a reference counter 
> > in the upper-level struct net."
> 
> ;-)  One of the nice things about both reference counting and RCU
> is that you can safely "sandwich" the protection regions in this
> way.  If you try it with pure locking, you are prone to deadlock.
> 
> Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Also applied, thanks everyone.
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