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[Xen-devel] Netback vif reference count mismatching in latest 3.11 kernels


After update of our network backend vm kernel to 3.11.9 I'm seeing trouble with netback vif close which seem related to the recent changes which separated vif disconnect and free; It seems that now multiple disconnect/connect cycles can happen without freeing and reallocing the netdev in the processes, which confuses the vif refcount.

vif refcount is initialized to 1 in xenvif_alloc. Then first xenvif_disconnect brings it back to 0, instead of 1 which would seem more reasonable (since its initialized to 1 in xenvif_alloc i would expect it to not be dropped to 0 until xenvif_free). Second xenvif_disconnect brings it to -1 and hangs. For us (xenclient XT) this happens when we hibernate linux guest, since linux hibernate is a complex beast which transitions the drivers to between close/connected states multiple times (i.e. first it suspends/closes the drivers to take memory snapshot, then resumes/reconnects the drivers to the actual writing of hibernate image to disk, then finally it closes them again to shutdown the system)

I've hacked the attached patch which fixes it (for us), is the approach taken 
there correct/upstreamable/reasonable? It does the following

* reset tx_irq to 0 after unbinding the irqs on disconnect - because xenvif_connect tests for it being 0 and will not reconnect if it's not reset
    * reacquire one reference to vif in disconnect(). This is because the 
      vif should be 1, as initialized in xenvif_alloc(), until the vif is 
freed. Otherwise multiple disconne
      and cause a hang. I imagine alternate way of fixing this could be to use 
"0" as the default
      refcnt in xenvif_alloc()

I believe we didn't experience this issue on previous kernel because vif disconnect was also freeing the vif and netdev, hence it was not possible to get xenvif_connect/xenvif_disconnect called multiple times between vif alloc/free.

Attachment: netback-refcnt-disconnect-fix
Description: Text document

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