[Xen-devel] virtio_net: Fix napi poll list corruption

[Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>]

David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
> After d75b1ade567ffab085e8adbbdacf0092d10cd09c (net: less interrupt
> masking in NAPI) the napi instance is removed from the per-cpu list
> prior to calling the n->poll(), and is only requeued if all of the
> budget was used.  This inadvertently broke netfront because netfront
> does not use NAPI correctly.

A similar bug exists in virtio_net.

-- >8 --
The commit d75b1ade567ffab085e8adbbdacf0092d10cd09c (net: less
interrupt masking in NAPI) breaks virtio_net in an insidious way.

It is now required that if the entire budget is consumed when poll
returns, the napi poll_list must remain empty.  However, like some
other drivers virtio_net tries to do a last-ditch check and if
there is more work it will call napi_schedule and then immediately
process some of this new work.  Should the entire budget be consumed
while processing such new work then we will violate the new caller
contract.

This patch fixes this by not touching any work when we reschedule
in virtio_net.

The worst part of this bug is that the list corruption causes other
napi users to be moved off-list.  In my case I was chasing a stall
in IPsec (IPsec uses netif_rx) and I only belatedly realised that it
was virtio_net which caused the stall even though the virtio_net
poll was still functioning perfectly after IPsec stalled.

Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index b8bd719..5ca9771 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -760,7 +760,6 @@ static int virtnet_poll(struct napi_struct *napi, int 
budget)
                container_of(napi, struct receive_queue, napi);
        unsigned int r, received = 0;
 
-again:
        received += virtnet_receive(rq, budget - received);
 
        /* Out of packets? */
@@ -771,7 +770,6 @@ again:
                    napi_schedule_prep(napi)) {
                        virtqueue_disable_cb(rq->vq);
                        __napi_schedule(napi);
-                       goto again;
                }
        }

Cheers,
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

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