On Thu, Apr 04, 2019 at 12:58:35PM +0200, Stefano Garzarella wrote:
> @@ -256,6 +257,7 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>       struct virtio_vsock_sock *vvs = vsk->trans;
>       struct virtio_vsock_pkt *pkt;
>       size_t bytes, total = 0;
> +     s64 free_space;

Why s64?  buf_alloc, fwd_cnt, and last_fwd_cnt are all u32.  fwd_cnt -
last_fwd_cnt <= buf_alloc is always true.

>       int err = -EFAULT;
>  
>       spin_lock_bh(&vvs->rx_lock);
> @@ -288,9 +290,15 @@ virtio_transport_stream_do_dequeue(struct vsock_sock 
> *vsk,
>       }
>       spin_unlock_bh(&vvs->rx_lock);
>  
> -     /* Send a credit pkt to peer */
> -     virtio_transport_send_credit_update(vsk, VIRTIO_VSOCK_TYPE_STREAM,
> -                                         NULL);
> +     /* We send a credit update only when the space available seen
> +      * by the transmitter is less than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE
> +      */
> +     free_space = vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);

Locking?  These fields should be accessed under tx_lock.

> +     if (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE) {
> +             virtio_transport_send_credit_update(vsk,
> +                                                 VIRTIO_VSOCK_TYPE_STREAM,
> +                                                 NULL);
> +     }

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