Re: [Xen-devel] Xen-unstable Linux 3.14-rc3 and 3.13 Network troubles "bisected"

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

Re-send shortened version...

> -----Original Message-----
> From: Sander Eikelenboom [mailto:linux@xxxxxxxxxxxxxx]
> Sent: 26 March 2014 16:54
> To: Paul Durrant
> Cc: Wei Liu; annie li; Zoltan Kiss; xen-devel@xxxxxxxxxxxxx; Ian Campbell; 
> linux-
> kernel; netdev@xxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Xen-unstable Linux 3.14-rc3 and 3.13 Network
> troubles "bisected"
> 
[snip]
> >>
> >> - When processing an SKB we end up in "xenvif_gop_frag_copy" while
> prod
> >> == cons ... but we still have bytes and size left ..
> >> - start_new_rx_buffer() has returned true ..
> >> - so we end up in get_next_rx_buffer
> >> - this does a RING_GET_REQUEST and ups cons ..
> >> - and we end up with a bad grant reference.
> >>
> >> Sometimes we are saved by the bell .. since additional slots have become
> >> free (you see cons become > prod in "get_next_rx_buffer" but shortly
> after
> >> that prod is increased ..
> >> just in time to not cause a overrun).
> >>
> 
> > Ah, but hang on... There's a BUG_ON meta_slots_used >
> max_slots_needed, so if we are overflowing the worst-case calculation then
> why is that BUG_ON not firing?
> 
> You mean:
>                 sco = (struct skb_cb_overlay *)skb->cb;
>                 sco->meta_slots_used = xenvif_gop_skb(skb, &npo);
>                 BUG_ON(sco->meta_slots_used > max_slots_needed);
> 
> in "get_next_rx_buffer" ?
> 

That code excerpt is from net_rx_action(),isn't it?

> I don't know .. at least now it doesn't crash dom0 and therefore not my
> complete machine and since tcp is recovering from a failed packet  :-)
> 

Well, if the code calculating max_slots_needed were underestimating then the 
BUG_ON() should fire. If it is not firing in your case then this suggests your 
problem lies elsewhere, or that meta_slots_used is not equal to the number of 
ring slots consumed.

> But probably because "npo->copy_prod++" seems to be used for the frags ..
> and it isn't added to  npo->meta_prod ?
> 

meta_slots_used is calculated as the value of meta_prod at return (from 
xenvif_gop_skb()) minus the value on entry , and if you look back up the code 
then you can see that meta_prod is incremented every time RING_GET_REQUEST() is 
evaluated. So, we must be consuming a slot without evaluating 
RING_GET_REQUEST() and I think that's exactly what's happening... Right at the 
bottom of xenvif_gop_frag_copy() req_cons is simply incremented in the case of 
a GSO. So the BUG_ON() is indeed off by one.

  Paul


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