Re: [Xen-devel] Intermittent xenvif_disconnect() hang on domU destroy

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 02/06/16 01:43, Ed Swierk wrote:
> I'm seeing the xenwatch kernel thread hang intermittently when
> destroying a domU on recent stable xen 4.5, with Linux 4.4.11 + grsec
> dom0.
> 
> The domU is created with a virtual network interface connected to a
> physical interface (ixgbevf) via an openvswitch virtual switch.
> 
> Everything works fine until the domain is destroyed. Once in a while,
> a few seconds after the domain goes away, xenwatch hangs in
> xenvif_disconnect(), calling kthread_stop() on a dealloc task.
> 
> I added a warning to xenvif_dealloc_kthread_should_stop() when
> kthread_should_stop() is true and queue->inflight_packets > 0,
> printing inflight_packets as well as stats.tx_zerocopy_*. Each time
> the hang occurs, inflight_packets == 1 and tx_zerocopy_sent ==
> tx_zerocopy_success + tx_zerocopy_fail + 1.
> 
> I also added a warning to xenvif_skb_zerocopy_complete() when
> queue->task is null. If I manually bring down the physical interface
> to which the vif was connected (ifconfig down), this somehow causes
> the last in-flight packet to be transmitted, and everything is
> unblocked.
> 
[...]
> 
> It's not clear to me whether the problem lies in netback, ixgbevf, or
> somewhere in between. Is the root cause ixgbevf hanging onto a skb for
> so long, and doing nothing with it until I bring the interface down,
> or is that a symptom of some other problem? Or is netback supposed to
> somehow flush in-flight transmit packets before it gets as far as
> xenvif_disconnect()?  Or should it forget about the in-flight packets
> since the interface is disappearing anyway?
> 
> Any clues would be appreciated.

netback can't flush in-flight packets because the network stack doesn't
provide a mechanism for this.  It can't forget about in-flight packets
because they have foreign (grant mapped) pages which must be unmapped
and the requests completed before the device can be CLOSED.

You need to investigate what is holding on to this skb.

David

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