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

Peter Zijlstra
2006-08-09T14:07:20Z

On Wed, 2006-08-09 at 15:19 +0200, Thomas Graf wrote:
> * Daniel Phillips <phillips@google.com> 2006-08-08 22:47
> > David Miller wrote:
> > >From: Daniel Phillips <phillips@google.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?