On Tue, Apr 13, 2021 at 10:29:03AM +0800, Jason Wang wrote:
>
> 在 2021/4/13 上午6:31, Michael S. Tsirkin 写道:
> > On Mon, Apr 12, 2021 at 06:03:50PM -0400, Michael S. Tsirkin wrote:
> > > I was working on the spurios interrupt problem and
> > > I noticed something weird.
> > >
> > > static int virtnet_poll_tx(struct napi_struct *napi, int budget)
> > > {
> > > struct send_queue *sq = container_of(napi, struct send_queue, napi);
> > > struct virtnet_info *vi = sq->vq->vdev->priv;
> > > unsigned int index = vq2txq(sq->vq);
> > > struct netdev_queue *txq;
> > >
> > > if (unlikely(is_xdp_raw_buffer_queue(vi, index))) {
> > > /* We don't need to enable cb for XDP */
> > > napi_complete_done(napi, 0);
> > > return 0;
> > > }
> > >
> > > txq = netdev_get_tx_queue(vi->dev, index);
> > > __netif_tx_lock(txq, raw_smp_processor_id());
> > > free_old_xmit_skbs(sq, true);
> > > __netif_tx_unlock(txq);
> > > virtqueue_napi_complete(napi, sq->vq, 0);
> > > if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
> > > netif_tx_wake_queue(txq);
> > > return 0;
> > > }
> > >
> > > So virtqueue_napi_complete is called when txq is not locked,
> > > thinkably start_xmit can happen right?
>
>
> Yes, I think so.
>
>
> > >
> > > Now virtqueue_napi_complete
> > >
> > > static void virtqueue_napi_complete(struct napi_struct *napi,
> > > struct virtqueue *vq, int processed)
> > > {
> > > int opaque;
> > >
> > > opaque = virtqueue_enable_cb_prepare(vq);
> > > if (napi_complete_done(napi, processed)) {
> > > if (unlikely(virtqueue_poll(vq, opaque)))
> > > virtqueue_napi_schedule(napi, vq);
> > > } else {
> > > virtqueue_disable_cb(vq);
> > > }
> > > }
> > >
> > >
> > > So it is calling virtqueue_enable_cb_prepare but tx queue
> > > could be running and can process things in parallel ...
> > > What gives? I suspect this corrupts the ring, and explains
> > > why we are seeing weird hangs with vhost packed ring ...
> > >
> > > Jason?
>
>
> It might cause the interrupt to be disabled unexpectedly I think.
>
>
> > >
> > >
> > > --
> > > MST
> > and wouldn't the following untested patch make sense then?
> >
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 82e520d2cb12..c23341b18eb5 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -1504,6 +1505,8 @@ static int virtnet_poll_tx(struct napi_struct *napi, int budget)
> > struct virtnet_info *vi = sq->vq->vdev->priv;
> > unsigned int index = vq2txq(sq->vq);
> > struct netdev_queue *txq;
> > + int opaque;
> > + bool done;
> > if (unlikely(is_xdp_raw_buffer_queue(vi, index))) {
> > /* We don't need to enable cb for XDP */
> > @@ -1514,9 +1517,26 @@ static int virtnet_poll_tx(struct napi_struct *napi, int budget)
> > txq = netdev_get_tx_queue(vi->dev, index);
> > __netif_tx_lock(txq, raw_smp_processor_id());
> > free_old_xmit_skbs(sq, true);
> > +
> > + opaque = virtqueue_enable_cb_prepare(sq->vq);
> > +
> > + done = napi_complete_done(napi, 0);
> > +
> > + if (!done)
> > + virtqueue_disable_cb(sq->vq);
> > +
> > __netif_tx_unlock(txq);
> > - virtqueue_napi_complete(napi, sq->vq, 0);
> > + if (done) {
> > + if (unlikely(virtqueue_poll(vq, opaque))) {
> > + if (napi_schedule_prep(napi)) {
> > + __netif_tx_lock(txq, raw_smp_processor_id());
> > + virtqueue_disable_cb(sq->vq);
> > + __netif_tx_unlock(txq);
> > + __napi_schedule(napi);
> > + }
> > + }
> > + }
> > if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
> > netif_tx_wake_queue(txq);
>
>
> I wonder why not simply protect the whole poll_tx with tx lock in this case?
>
> Thanks
>
Well it takes __netif_tx_lock internally does it not? Sounds like a
deadlock to me ..,
--
MST
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