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

[Jason Wang]

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 <m...@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);