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

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Date:   Fri, 28 May 2021 18:25:11 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>, Wei Wang <weiwan@...gle.com>,
        David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH v3 1/4] virtio_net: move tx vq operation under tx queue lock

On Wed, May 26, 2021 at 11:41 PM Jason Wang <jasowang@...hat.com> 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 <mst@...hat.com>
> > ---
> >   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?

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

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