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

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.

Fixes: b92f1e6751a6 ("virtio-net: transmit napi")
Signed-off-by: Michael S. Tsirkin <mst@redhat.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))) {
+			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);
+			}
+		}
+	}