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

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Date:   Tue, 13 Apr 2021 16:54:42 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Jakub Kicinski <kuba@...nel.org>, Wei Wang <weiwan@...gle.com>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Willem de Bruijn <willemb@...gle.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH RFC v2 3/4] virtio_net: move tx vq operation under tx
 queue lock


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@...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 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);