Re: [PATCH net-next v4 3/4] virtio-net: Map NAPIs to queues

[Joe Damato <jdamato@fastly.com>]

On Wed, Feb 26, 2025 at 01:48:50PM +0800, Jason Wang wrote:
> On Tue, Feb 25, 2025 at 10:05 AM Joe Damato <jdamato@fastly.com> wrote:
> >
> > Use netif_queue_set_napi to map NAPIs to queue IDs so that the mapping
> > can be accessed by user apps, taking care to hold RTNL as needed.
> 
> I may miss something but I wonder whether letting the caller hold the
> lock is better.

Hmm...

Double checking all the paths over again, here's what I see:
  - refill_work, delayed work that needs RTNL so this change seems
    right?

  - virtnet_disable_queue_pair, called from virtnet_open and
    virtnet_close. When called via NDO these are safe and hold RTNL,
    but they can be called from power management and need RTNL.

  - virtnet_enable_queue_pair called from virtnet_open, safe when
    used via NDO but needs RTNL when used via power management.

  - virtnet_rx_pause called in both paths as you mentioned, one
    which needs RTNL and one which doesn't.

I think there are a couple ways to fix this:

  1. Edit this patch to remove the virtnet_queue_set_napi helper,
     and call netif_queue_set_napi from the napi_enable and
     napi_disable helpers directly. Modify code calling into these
     paths to hold rtnl (or not) as described above.

  2. Modify virtnet_enable_queue_pair, virtnet_disable_queue_pair,
     and virtnet_rx_pause to take a "bool need_rtnl" as an a
     function argument and pass that through.

I'm not sure which is cleaner and I do not have a preference.

Can you let me know which you prefer? I am happy to implement either
one for the next revision.

[...]

> > ---
> >  drivers/net/virtio_net.c | 73 ++++++++++++++++++++++++++++------------
> >  1 file changed, 52 insertions(+), 21 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index e578885c1093..13bb4a563073 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -2807,6 +2807,20 @@ static void skb_recv_done(struct virtqueue *rvq)
> >         virtqueue_napi_schedule(&rq->napi, rvq);
> >  }
> >
> > +static void virtnet_queue_set_napi(struct net_device *dev,
> > +                                  struct napi_struct *napi,
> > +                                  enum netdev_queue_type q_type, int qidx,
> > +                                  bool need_rtnl)
> > +{
> > +       if (need_rtnl)
> > +               rtnl_lock();
> > +
> > +       netif_queue_set_napi(dev, qidx, q_type, napi);
> > +
> > +       if (need_rtnl)
> > +               rtnl_unlock();
> > +}
> > +
> >  static void virtnet_napi_do_enable(struct virtqueue *vq,
> >                                    struct napi_struct *napi)
> >  {
> > @@ -2821,15 +2835,21 @@ static void virtnet_napi_do_enable(struct virtqueue *vq,
> >         local_bh_enable();
> >  }
> >
> > -static void virtnet_napi_enable(struct receive_queue *rq)
> > +static void virtnet_napi_enable(struct receive_queue *rq, bool need_rtnl)
> >  {
> > +       struct virtnet_info *vi = rq->vq->vdev->priv;
> > +       int qidx = vq2rxq(rq->vq);
> > +
> >         virtnet_napi_do_enable(rq->vq, &rq->napi);
> > +       virtnet_queue_set_napi(vi->dev, &rq->napi,
> > +                              NETDEV_QUEUE_TYPE_RX, qidx, need_rtnl);
> >  }
> >
> > -static void virtnet_napi_tx_enable(struct send_queue *sq)
> > +static void virtnet_napi_tx_enable(struct send_queue *sq, bool need_rtnl)
> >  {
> >         struct virtnet_info *vi = sq->vq->vdev->priv;
> >         struct napi_struct *napi = &sq->napi;
> > +       int qidx = vq2txq(sq->vq);
> >
> >         if (!napi->weight)
> >                 return;
> > @@ -2843,20 +2863,31 @@ static void virtnet_napi_tx_enable(struct send_queue *sq)
> >         }
> >
> >         virtnet_napi_do_enable(sq->vq, napi);
> > +       virtnet_queue_set_napi(vi->dev, napi, NETDEV_QUEUE_TYPE_TX, qidx,
> > +                              need_rtnl);
> >  }
> >
> > -static void virtnet_napi_tx_disable(struct send_queue *sq)
> > +static void virtnet_napi_tx_disable(struct send_queue *sq, bool need_rtnl)
> >  {
> > +       struct virtnet_info *vi = sq->vq->vdev->priv;
> >         struct napi_struct *napi = &sq->napi;
> > +       int qidx = vq2txq(sq->vq);
> >
> > -       if (napi->weight)
> > +       if (napi->weight) {
> > +               virtnet_queue_set_napi(vi->dev, NULL, NETDEV_QUEUE_TYPE_TX,
> > +                                      qidx, need_rtnl);
> >                 napi_disable(napi);
> > +       }
> >  }
> >
> > -static void virtnet_napi_disable(struct receive_queue *rq)
> > +static void virtnet_napi_disable(struct receive_queue *rq, bool need_rtnl)
> >  {
> > +       struct virtnet_info *vi = rq->vq->vdev->priv;
> >         struct napi_struct *napi = &rq->napi;
> > +       int qidx = vq2rxq(rq->vq);
> >
> > +       virtnet_queue_set_napi(vi->dev, NULL, NETDEV_QUEUE_TYPE_TX, qidx,
> > +                              need_rtnl);
> >         napi_disable(napi);
> >  }
> >
> > @@ -2870,9 +2901,9 @@ static void refill_work(struct work_struct *work)
> >         for (i = 0; i < vi->curr_queue_pairs; i++) {
> >                 struct receive_queue *rq = &vi->rq[i];
> >
> > -               virtnet_napi_disable(rq);
> > +               virtnet_napi_disable(rq, true);
> >                 still_empty = !try_fill_recv(vi, rq, GFP_KERNEL);
> > -               virtnet_napi_enable(rq);
> > +               virtnet_napi_enable(rq, true);
> >
> >                 /* In theory, this can happen: if we don't get any buffers in
> >                  * we will *never* try to fill again.
> > @@ -3069,8 +3100,8 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
> >
> >  static void virtnet_disable_queue_pair(struct virtnet_info *vi, int qp_index)
> >  {
> > -       virtnet_napi_tx_disable(&vi->sq[qp_index]);
> > -       virtnet_napi_disable(&vi->rq[qp_index]);
> > +       virtnet_napi_tx_disable(&vi->sq[qp_index], false);
> > +       virtnet_napi_disable(&vi->rq[qp_index], false);
> >         xdp_rxq_info_unreg(&vi->rq[qp_index].xdp_rxq);
> >  }
> >
> > @@ -3089,8 +3120,8 @@ static int virtnet_enable_queue_pair(struct virtnet_info *vi, int qp_index)
> >         if (err < 0)
> >                 goto err_xdp_reg_mem_model;
> >
> > -       virtnet_napi_enable(&vi->rq[qp_index]);
> > -       virtnet_napi_tx_enable(&vi->sq[qp_index]);
> > +       virtnet_napi_enable(&vi->rq[qp_index], false);
> > +       virtnet_napi_tx_enable(&vi->sq[qp_index], false);
> >
> >         return 0;
> >
> > @@ -3342,7 +3373,7 @@ static void virtnet_rx_pause(struct virtnet_info *vi, struct receive_queue *rq)
> >         bool running = netif_running(vi->dev);
> >
> >         if (running) {
> > -               virtnet_napi_disable(rq);
> > +               virtnet_napi_disable(rq, true);
> 
> During the resize, the rtnl lock has been held on the ethtool path
> 
>         rtnl_lock();
>         rc = __dev_ethtool(net, ifr, useraddr, ethcmd, state);
>         rtnl_unlock();
> 
> virtnet_rx_resize()
>     virtnet_rx_pause()
> 
> and in the case of XSK binding, I see ASSERT_RTNL in xp_assign_dev() at least.

Thanks for catching this. I re-read all the paths and I think I've
outlined a few other issues above.

Please let me know which of the proposed methods above you'd like me
to implement to get this merged.

Thanks.

---
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