Re: [PATCH] af_packet: Don't use skb after dev_queue_xmit()

[Michael Breuer]

On 1/7/2010 3:21 AM, Jarek Poplawski wrote:
> On Thu, Jan 07, 2010 at 02:55:20AM -0500, Michael Breuer wrote:
>    
> > On 1/7/2010 2:47 AM, Jarek Poplawski wrote:
> >      
> > > On Thu, Jan 07, 2010 at 02:20:22AM -0500, Michael Breuer wrote:
> > >
> > >        
> > > > ...
> > > > Reapplied a couple of earlier patches - still can't do jumbo frames, but
> > > > the rx errors are gone and speed has improved. Too early to assure that
> > > > it's stable.
> > > >
> > > > Patches that seem to fix the rx drops (all from Stephen):
> > > > 1) Patch change to tx_init
> > > > 2) Patch to lock netif_device_detach
> > > > 3) Patch to sky2_tx_complete to add netif_device_present test
> > > > Also in the mix: Jarek's alternative 2
> > > >
> > > >          
> > > BTW, the main difference between alt. 1 and 2 is error notification:
> > > alternative 2 doesn't hide some (most) of drops, so, dependending on
> > > app, there might be more and faster retransmits. (I don't know what
> > > apps used by you (other than dhcp) can depend so much on this.)
> > >
> > >
> > >        
> > Unless I misread the code, I think that in some cases e skb is actually
> > freed if the cfq (among others perhaps) scheduler returns an error on
> > enqueue (flow control perhaps).  Thus with alternative 1, it is possible
> > that the skb is acted upon after being freed - this would be consistent
> > with the DMAR errors I saw.
> >      
> I can't see your point: could you give some scenario?
>
> Jarek P.
>    
With NET_CLS_ACT set, net_dev_enqueue can return an error after freeing 
the skb. Alternative 1 disregards the error and assumes the skb is still 
valid. The original code and alternative 2 exit the loop assuming the 
skb has been freed.
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