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

[Joe Damato <jdamato@fastly.com>]

On Thu, Feb 27, 2025 at 12:18:33PM +0800, Jason Wang wrote:
> On Thu, Feb 27, 2025 at 6:13 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Wed, Feb 26, 2025 at 03:27:42PM -0500, Joe Damato wrote:
> > > On Wed, Feb 26, 2025 at 01:08:49PM -0500, Joe Damato wrote:
> > > > On Wed, Feb 26, 2025 at 01:03:09PM -0500, Joe Damato wrote:
> > > > > 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.
> > > >
> > > > Sorry, I missed more paths:
> > > >
> > > >     - virtnet_rx_resume
> > > >     - virtnet_tx_pause and virtnet_tx_resume
> > > >
> > > > which are similar to path you mentioned (virtnet_rx_pause) and need
> > > > rtnl in one of two different paths.
> > > >
> > > > Let me know if I missed any paths and what your preferred way to fix
> > > > this would be?
> > > >
> > > > I think both options below are possible and I have no strong
> > > > preference.
> > >
> > > OK, my apologies. I read your message and the code wrong. Sorry for
> > > the back-to-back emails from me.
> > >
> > > Please ignore my message above... I think after re-reading the code,
> > > here's where I've arrived:
> > >
> > >   - refill_work needs to hold RTNL (as in the existing patch)
> > >
> > >   - virtnet_rx_pause, virtnet_rx_resume, virtnet_tx_pause,
> > >     virtnet_tx_resume -- all do NOT need to hold RTNL because it is
> > >     already held in the ethtool resize path and the XSK path, as you
> > >     explained, but I mis-read (sorry).
> > >
> > >   - virtnet_disable_queue_pair and virtnet_enable_queue_pair both
> > >     need to hold RTNL only when called via power management, but not
> > >     when called via ndo_open or ndo_close
> > >
> > > Is my understanding correct and does it match your understanding?
> > >
> > > If so, that means there are two issues:
> > >
> > >   1. Fixing the hardcoded bools in rx_pause, rx_resume, tx_pause,
> > >      tx_resume (all should be false, RTNL is not needed).
> > >
> > >   2. Handling the power management case which calls virtnet_open and
> > >      virtnet_close.
> > >
> > > I made a small diff included below as an example of a possible
> > > solution:
> > >
> > >   1. Modify virtnet_disable_queue_pair and virtnet_enable_queue_pair
> > >      to take a "bool need_rtnl" and pass it through to the helpers
> > >      they call.
> > >
> > >   2. Create two helpers, virtnet_do_open and virt_do_close both of
> > >      which take struct net_device *dev, bool need_rtnl. virtnet_open
> > >      and virtnet_close are modified to call the helpers and pass
> > >      false for need_rtnl. The power management paths call the
> > >      helpers and pass true for need_rtnl. (fixes issue 2 above)
> > >
> > >   3. Fix the bools for rx_pause, rx_resume, tx_pause, tx_resume to
> > >      pass false since all paths that I could find that lead to these
> > >      functions hold RTNL. (fixes issue 1 above)
> > >
> > > See the diff below (which can be applied on top of patch 3) to see
> > > what it looks like.
> > >
> > > If you are OK with this approach, I will send a v5 where patch 3
> > > includes the changes shown in this diff.
> > >
> > > Please let me know what you think:
> >
> >
> >
> > Looks ok I think.
> >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 13bb4a563073..76ecb8f3ce9a 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -3098,14 +3098,16 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
> > >       return received;
> > >  }
> > >
> > > -static void virtnet_disable_queue_pair(struct virtnet_info *vi, int qp_index)
> > > +static void virtnet_disable_queue_pair(struct virtnet_info *vi, int qp_index,
> > > +                                    bool need_rtnl)
> > >  {
> > > -     virtnet_napi_tx_disable(&vi->sq[qp_index], false);
> > > -     virtnet_napi_disable(&vi->rq[qp_index], false);
> > > +     virtnet_napi_tx_disable(&vi->sq[qp_index], need_rtnl);
> > > +     virtnet_napi_disable(&vi->rq[qp_index], need_rtnl);
> > >       xdp_rxq_info_unreg(&vi->rq[qp_index].xdp_rxq);
> > >  }
> > >
> > > -static int virtnet_enable_queue_pair(struct virtnet_info *vi, int qp_index)
> > > +static int virtnet_enable_queue_pair(struct virtnet_info *vi, int qp_index,
> > > +                                  bool need_rtnl)
> > >  {
> > >       struct net_device *dev = vi->dev;
> > >       int err;
> > > @@ -3120,8 +3122,8 @@ 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], false);
> > > -     virtnet_napi_tx_enable(&vi->sq[qp_index], false);
> > > +     virtnet_napi_enable(&vi->rq[qp_index], need_rtnl);
> > > +     virtnet_napi_tx_enable(&vi->sq[qp_index], need_rtnl);
> > >
> > >       return 0;
> > >
> > > @@ -3156,7 +3158,7 @@ static void virtnet_update_settings(struct virtnet_info *vi)
> > >               vi->duplex = duplex;
> > >  }
> > >
> > > -static int virtnet_open(struct net_device *dev)
> > > +static int virtnet_do_open(struct net_device *dev, bool need_rtnl)
> > >  {
> > >       struct virtnet_info *vi = netdev_priv(dev);
> > >       int i, err;
> > > @@ -3169,7 +3171,7 @@ static int virtnet_open(struct net_device *dev)
> > >                       if (!try_fill_recv(vi, &vi->rq[i], GFP_KERNEL))
> > >                               schedule_delayed_work(&vi->refill, 0);
> > >
> > > -             err = virtnet_enable_queue_pair(vi, i);
> > > +             err = virtnet_enable_queue_pair(vi, i, need_rtnl);
> > >               if (err < 0)
> > >                       goto err_enable_qp;
> > >       }
> > > @@ -3190,13 +3192,18 @@ static int virtnet_open(struct net_device *dev)
> > >       cancel_delayed_work_sync(&vi->refill);
> > >
> > >       for (i--; i >= 0; i--) {
> > > -             virtnet_disable_queue_pair(vi, i);
> > > +             virtnet_disable_queue_pair(vi, i, need_rtnl);
> > >               virtnet_cancel_dim(vi, &vi->rq[i].dim);
> > >       }
> > >
> > >       return err;
> > >  }
> > >
> > > +static int virtnet_open(struct net_device *dev)
> > > +{
> > > +     return virtnet_do_open(dev, false);
> > > +}
> > > +
> > >  static int virtnet_poll_tx(struct napi_struct *napi, int budget)
> > >  {
> > >       struct send_queue *sq = container_of(napi, struct send_queue, napi);
> > > @@ -3373,7 +3380,7 @@ 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, true);
> > > +             virtnet_napi_disable(rq, false);
> > >               virtnet_cancel_dim(vi, &rq->dim);
> > >       }
> > >  }
> > > @@ -3386,7 +3393,7 @@ static void virtnet_rx_resume(struct virtnet_info *vi, struct receive_queue *rq)
> > >               schedule_delayed_work(&vi->refill, 0);
> > >
> > >       if (running)
> > > -             virtnet_napi_enable(rq, true);
> > > +             virtnet_napi_enable(rq, false);
> > >  }
> > >
> > >  static int virtnet_rx_resize(struct virtnet_info *vi,
> > > @@ -3415,7 +3422,7 @@ static void virtnet_tx_pause(struct virtnet_info *vi, struct send_queue *sq)
> > >       qindex = sq - vi->sq;
> > >
> > >       if (running)
> > > -             virtnet_napi_tx_disable(sq, true);
> > > +             virtnet_napi_tx_disable(sq, false);
> > >
> > >       txq = netdev_get_tx_queue(vi->dev, qindex);
> > >
> > > @@ -3449,7 +3456,7 @@ static void virtnet_tx_resume(struct virtnet_info *vi, struct send_queue *sq)
> > >       __netif_tx_unlock_bh(txq);
> > >
> > >       if (running)
> > > -             virtnet_napi_tx_enable(sq, true);
> > > +             virtnet_napi_tx_enable(sq, false);
> 
> Instead of this, it looks to me it would be much simpler if we can
> just hold the rtnl lock in freeze/restore.

I disagree.

Holding RTNL for all of open and close instead of just the 1 API
call that needs it has the possibility of introducing other lock
ordering bugs now or in the future.

We only need RTNL for 1 API, why hold it for all of open or close?