Subject: Re: [PATCH RFC v2 3/4] virtio_net: move tx vq operation under tx queue lock

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 13 Apr 2021 10:02:55 -0400

On Tue, Apr 13, 2021 at 04:54:42PM +0800, Jason Wang wrote:
> 
> 在 2021/4/13 下午1:47, 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.
> > As a fix move everything that deals with the vq to under tx lock.
> > 
> > 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 16d5abed582c..460ccdbb840e 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -1505,6 +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 */
> > @@ -1514,10 +1516,28 @@ 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);
> 
> 
> So I wonder why not simply move __netif_tx_unlock() after
> virtqueue_napi_complete()?
> 
> Thanks
> 

Because that calls tx poll which also takes tx lock internally ...

> > +	if (done) {
> > +		if (unlikely(virtqueue_poll(sq->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);