Subject: Re: [PATCH v3 1/4] virtio_net: move tx vq operation under tx queue lock

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 9 Jun 2021 18:03:59 -0400

On Fri, May 28, 2021 at 06:25:11PM -0400, Willem de Bruijn wrote:
> On Wed, May 26, 2021 at 11:41 PM Jason Wang <[email-masked]> wrote:
> >
> >
> > 在 2021/5/26 下午4:24, Michael S. Tsirkin 写道:
> > > It's unsafe to operate a vq from multiple threads.
> > > Unfortunately this is exactly what we do when invoking
> > > clean tx poll from rx napi.
> > > Same happens with napi-tx even without the
> > > opportunistic cleaning from the receive interrupt: that races
> > > with processing the vq in start_xmit.
> > >
> > > As a fix move everything that deals with the vq to under tx lock.
> 
> This patch also disables callbacks during free_old_xmit_skbs
> processing on tx interrupt. Should that be a separate commit, with its
> own explanation?
> > >
> > > Fixes: b92f1e6751a6 ("virtio-net: transmit napi")
> > > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > > ---
> > >   drivers/net/virtio_net.c | 22 +++++++++++++++++++++-
> > >   1 file changed, 21 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index ac0c143f97b4..12512d1002ec 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -1508,6 +1508,8 @@ 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 */
> > > @@ -1517,10 +1519,28 @@ 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());
> > > +     virtqueue_disable_cb(sq->vq);
> > >       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(sq->vq, opaque))) {
> 
> Should this also be inside the lock, as it operates on vq?

No vq poll is ok outside of locks, it's atomic.

> Is there anything that is not allowed to run with the lock held?
> > > +                     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);
> > > +                     }
> > > +             }
> > > +     }
> >
> >
> > Interesting, this looks like somehwo a open-coded version of
> > virtqueue_napi_complete(). I wonder if we can simply keep using
> > virtqueue_napi_complete() by simply moving the __netif_tx_unlock() after
> > that:
> >
> > netif_tx_lock(txq);
> > free_old_xmit_skbs(sq, true);
> > virtqueue_napi_complete(napi, sq->vq, 0);
> > __netif_tx_unlock(txq);
> 
> Agreed. And subsequent block
> 
>        if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
>                netif_tx_wake_queue(txq);
> 
> as well

Yes I thought I saw something here that can't be called with tx lock
held but I no longer see it. Will do.

> >
> > Thanks
> >
> >
> > >
> > >       if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
> > >               netif_tx_wake_queue(txq);
> >