[PATCH RFC v2 4/4] virtio_net: move txq wakeups under tx q lock

[Michael S. Tsirkin]

We currently check num_free outside tx q lock
which is unsafe: new packets can arrive meanwhile
and there won't be space in the queue.
Thus a spurious queue wakeup causing overhead
and even packet drops.

Move the check under the lock to fix that.

Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
---
 drivers/net/virtio_net.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 460ccdbb840e..febaf55ec1f6 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1431,11 +1431,12 @@ static void virtnet_poll_cleantx(struct receive_queue 
*rq)
        if (__netif_tx_trylock(txq)) {
                virtqueue_disable_cb(sq->vq);
                free_old_xmit_skbs(sq, true);
+
+               if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
+                       netif_tx_wake_queue(txq);
+
                __netif_tx_unlock(txq);
        }
-
-       if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
-               netif_tx_wake_queue(txq);
 }
 
 static int virtnet_poll(struct napi_struct *napi, int budget)
@@ -1519,6 +1520,9 @@ static int virtnet_poll_tx(struct napi_struct *napi, int 
budget)
        virtqueue_disable_cb(sq->vq);
        free_old_xmit_skbs(sq, true);
 
+       if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
+               netif_tx_wake_queue(txq);
+
        opaque = virtqueue_enable_cb_prepare(sq->vq);
 
        done = napi_complete_done(napi, 0);
@@ -1539,9 +1543,6 @@ static int virtnet_poll_tx(struct napi_struct *napi, int 
budget)
                }
        }
 
-       if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
-               netif_tx_wake_queue(txq);
-
        return 0;
 }
 
-- 
MST