virtio-net: locking in tx napi

["Michael S. Tsirkin" <mst@redhat.com>]

I was working on the spurios interrupt problem and
I noticed something weird.

static int virtnet_poll_tx(struct napi_struct *napi, int budget)
{       
        struct send_queue *sq = container_of(napi, struct send_queue, napi);
        struct virtnet_info *vi = sq->vq->vdev->priv;
        unsigned int index = vq2txq(sq->vq);
        struct netdev_queue *txq;

        if (unlikely(is_xdp_raw_buffer_queue(vi, index))) {
                /* We don't need to enable cb for XDP */
                napi_complete_done(napi, 0);
                return 0;
        }

        txq = netdev_get_tx_queue(vi->dev, index);
        __netif_tx_lock(txq, raw_smp_processor_id());
        free_old_xmit_skbs(sq, true);
        __netif_tx_unlock(txq);
        
        virtqueue_napi_complete(napi, sq->vq, 0);
        
        if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
                netif_tx_wake_queue(txq);
        
        return 0;
}       

So virtqueue_napi_complete is called when txq is not locked,
thinkably start_xmit can happen right?

Now virtqueue_napi_complete

static void virtqueue_napi_complete(struct napi_struct *napi,
                                    struct virtqueue *vq, int processed)
{
        int opaque;

        opaque = virtqueue_enable_cb_prepare(vq);
        if (napi_complete_done(napi, processed)) {
                if (unlikely(virtqueue_poll(vq, opaque)))
                        virtqueue_napi_schedule(napi, vq);
        } else {
                virtqueue_disable_cb(vq);
        }
}


So it is calling virtqueue_enable_cb_prepare but tx queue
could be running and can process things in parallel ...
What gives? I suspect this corrupts the ring, and explains
why we are seeing weird hangs with vhost packed ring ...

Jason?


-- 
MST

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