Subject: Re: [PATCH net-next v5 2/3] virtio/vsock: send credit update during setting SO_RCVLOWAT

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 30 Nov 2023 08:42:49 -0500

On Thu, Nov 30, 2023 at 04:08:39PM +0300, Arseniy Krasnov wrote:
> Send credit update message when SO_RCVLOWAT is updated and it is bigger
> than number of bytes in rx queue. It is needed, because 'poll()' will
> wait until number of bytes in rx queue will be not smaller than
> SO_RCVLOWAT, so kick sender to send more data. Otherwise mutual hungup
> for tx/rx is possible: sender waits for free space and receiver is
> waiting data in 'poll()'.
> 
> Signed-off-by: Arseniy Krasnov <[email-masked]>
> ---
>  Changelog:
>  v1 -> v2:
>   * Update commit message by removing 'This patch adds XXX' manner.
>   * Do not initialize 'send_update' variable - set it directly during
>     first usage.
>  v3 -> v4:
>   * Fit comment in 'virtio_transport_notify_set_rcvlowat()' to 80 chars.
>  v4 -> v5:
>   * Do not change callbacks order in transport structures.
> 
>  drivers/vhost/vsock.c                   |  1 +
>  include/linux/virtio_vsock.h            |  1 +
>  net/vmw_vsock/virtio_transport.c        |  1 +
>  net/vmw_vsock/virtio_transport_common.c | 27 +++++++++++++++++++++++++
>  net/vmw_vsock/vsock_loopback.c          |  1 +
>  5 files changed, 31 insertions(+)
> 
> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> index f75731396b7e..4146f80db8ac 100644
> --- a/drivers/vhost/vsock.c
> +++ b/drivers/vhost/vsock.c
> @@ -451,6 +451,7 @@ static struct virtio_transport vhost_transport = {
>  		.notify_buffer_size       = virtio_transport_notify_buffer_size,
>  
>  		.read_skb = virtio_transport_read_skb,
> +		.notify_set_rcvlowat      = virtio_transport_notify_set_rcvlowat
>  	},
>  
>  	.send_pkt = vhost_transport_send_pkt,
> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> index ebb3ce63d64d..c82089dee0c8 100644
> --- a/include/linux/virtio_vsock.h
> +++ b/include/linux/virtio_vsock.h
> @@ -256,4 +256,5 @@ void virtio_transport_put_credit(struct virtio_vsock_sock *vvs, u32 credit);
>  void virtio_transport_deliver_tap_pkt(struct sk_buff *skb);
>  int virtio_transport_purge_skbs(void *vsk, struct sk_buff_head *list);
>  int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t read_actor);
> +int virtio_transport_notify_set_rcvlowat(struct vsock_sock *vsk, int val);
>  #endif /* _LINUX_VIRTIO_VSOCK_H */
> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> index af5bab1acee1..8007593a3a93 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -539,6 +539,7 @@ static struct virtio_transport virtio_transport = {
>  		.notify_buffer_size       = virtio_transport_notify_buffer_size,
>  
>  		.read_skb = virtio_transport_read_skb,
> +		.notify_set_rcvlowat      = virtio_transport_notify_set_rcvlowat
>  	},
>  
>  	.send_pkt = virtio_transport_send_pkt,
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index f6dc896bf44c..1cb556ad4597 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -1684,6 +1684,33 @@ int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t recv_acto
>  }
>  EXPORT_SYMBOL_GPL(virtio_transport_read_skb);
>  
> +int virtio_transport_notify_set_rcvlowat(struct vsock_sock *vsk, int val)
> +{
> +	struct virtio_vsock_sock *vvs = vsk->trans;
> +	bool send_update;
> +
> +	spin_lock_bh(&vvs->rx_lock);
> +
> +	/* If number of available bytes is less than new SO_RCVLOWAT value,
> +	 * kick sender to send more data, because sender may sleep in its
> +	 * 'send()' syscall waiting for enough space at our side.
> +	 */
> +	send_update = vvs->rx_bytes < val;
> +
> +	spin_unlock_bh(&vvs->rx_lock);
> +
> +	if (send_update) {
> +		int err;
> +
> +		err = virtio_transport_send_credit_update(vsk);
> +		if (err < 0)
> +			return err;
> +	}
> +
> +	return 0;
> +}

I find it strange that this will send a credit update
even if nothing changed since this was called previously.
I'm not sure whether this is a problem protocol-wise,
but it certainly was not envisioned when the protocol was
built. WDYT?

> +EXPORT_SYMBOL_GPL(virtio_transport_notify_set_rcvlowat);
> +
>  MODULE_LICENSE("GPL v2");
>  MODULE_AUTHOR("Asias He");
>  MODULE_DESCRIPTION("common code for virtio vsock");
> diff --git a/net/vmw_vsock/vsock_loopback.c b/net/vmw_vsock/vsock_loopback.c
> index 048640167411..9f4b814fbbc7 100644
> --- a/net/vmw_vsock/vsock_loopback.c
> +++ b/net/vmw_vsock/vsock_loopback.c
> @@ -98,6 +98,7 @@ static struct virtio_transport loopback_transport = {
>  		.notify_buffer_size       = virtio_transport_notify_buffer_size,
>  
>  		.read_skb = virtio_transport_read_skb,
> +		.notify_set_rcvlowat      = virtio_transport_notify_set_rcvlowat
>  	},
>  
>  	.send_pkt = vsock_loopback_send_pkt,
> -- 
> 2.25.1