Re: [PATCH 0/2] pktgen: Clone skb to avoid corruption of skbs in ndo_start_xmit methods (v3)

Neil Horman
2011-08-22T00:28:08Z

On Wed, Aug 17, 2011 at 04:07:17PM +0100, Ben Hutchings wrote:
> On Tue, 2011-07-26 at 12:05 -0400, Neil Horman wrote:
> > Ok, after considering all your comments, Dave suggested this as an alternate
> > approach:
> >
> > 1) We create a new priv_flag, IFF_SKB_TX_SHARED, to identify drivers capable of
> > handling shared skbs. Default is to not set this flag
> >
> > 2) Modify ether_setup to enable this flag, under the assumption that any driver
> > calling this function is initalizing a real ethernet device and as such can
> > handle shared skbs since they don't tend to store state in the skb struct.
> > Pktgen can then query this flag when a user script attempts to issue the
> > clone_skb command and decide if it is to be alowed or not.
> [...]
>
> A bunch of Ethernet drivers do skb_pad() or skb_padto() in their
> ndo_start_xmit implementations, either to avoid hardware bugs or because
> the MAC doesn't automatically pad to the minimum frame length. This
> presumably means they can't generally handle shared skbs, though in the
> specific case of pktgen it should be safe as long as a single skb is not
> submitted by multiple threads at once.
>
Agreed, given that pktgen is doing skb sharing in a serialized manner (i.e. one
thread of execution increasing skb->users rather than in multiple threads), the
skb_pad[to] cases are safe. Are there cases in which shared skbs are
transmitted in parallel threads that we need to check for?
Neil

> Ben.
>
> --
> Ben Hutchings, Staff Engineer, Solarflare
> Not speaking for my employer; that's the marketing department's job.
> They asked us to note that Solarflare product names are trademarked.
>
>