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

Jason Wang
2021-04-13T08:54:59Z

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 <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 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

> + 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);