linux-kernel - Re: [RFC][PATCH 2/9] deadlock prevention core

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Date:	Wed, 09 Aug 2006 16:07:20 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	Daniel Phillips <phillips@...gle.com>,
	David Miller <davem@...emloft.net>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [RFC][PATCH 2/9] deadlock prevention core

On Wed, 2006-08-09 at 15:19 +0200, Thomas Graf wrote:
> * Daniel Phillips <phillips@...gle.com> 2006-08-08 22:47
> > David Miller wrote:
> > >From: Daniel Phillips <phillips@...gle.com>
> >  >>Can you please characterize the conditions under which skb->dev changes
> > >>after the alloc?  Are there writings on this subtlety?
> > >
> > >The packet scheduler and classifier can redirect packets to different
> > >devices, and can the netfilter layer.
> > >
> > >The setting of skb->dev is wholly transient and you cannot rely upon
> > >it to be the same as when you set it on allocation.
> > >
> > >Even simple things like the bonding device change skb->dev on every
> > >receive.
> > 
> > Thankyou, this is easily fixed.
> 
> It's not that simple, in order to just fix the most obvious case
> being packet forwarding when skb->dev changes its meaning from
> device the packet is coming from to device the packet will be leaving
> on is difficult.
> 
> You can't unreserve at that point so you need to keep the original
> skb->dev. Since the packet is mostly likely queued before freeing
> you will lose the refcnt on the original skb->dev. Keeping a
> refcnt just for this memalloc stuff is out of question. Even keeping
> the ifindex on a best effort basis is unlikely an option, sk_buff is
> way overweight already.

I think Daniel was thinking of adding struct net_device *
sk_buff::alloc_dev,
I know I was after reading the first few mails. However if adding a
field 
there is strict no-no....

/me takes a look at struct sk_buff

Hmm, what does sk_buff::input_dev do? That seems to store the initial
device?


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