Re: [PATCH] vsock/virtio: use GFP_ATOMIC under RCU read lock

["Gupta, Pankaj" <pankaj.gupta@amd.com>]

> virtio_transport_send_pkt in now called on transport fast path,
> under RCU read lock. In that case, we have a bug: virtio_add_sgs
> is called with GFP_KERNEL, and might sleep.
> 
> Pass the gfp flags as an argument, and use GFP_ATOMIC on
> the fast path.
> 
> Link: https://lore.kernel.org/all/hfcr2aget2zojmqpr4uhlzvnep4vgskblx5b6xf2ddosbsrke7@nt34bxgp7j2x
> Fixes: efcd71af38be ("vsock/virtio: avoid queuing packets when intermediate queue is empty")
> Reported-by: Christian Brauner <brauner@kernel.org>
> Cc: Stefano Garzarella <sgarzare@redhat.com>
> Cc: Luigi Leonardi <luigi.leonardi@outlook.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>

> ---
> 
> Lightly tested. Christian, could you pls confirm this fixes the problem
> for you? Stefano, it's a holiday here - could you pls help test!
> Thanks!
> 
> 
>   net/vmw_vsock/virtio_transport.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> index f992f9a216f0..0cd965f24609 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -96,7 +96,7 @@ static u32 virtio_transport_get_local_cid(void)
>   
>   /* Caller need to hold vsock->tx_lock on vq */
>   static int virtio_transport_send_skb(struct sk_buff *skb, struct virtqueue *vq,
> -				     struct virtio_vsock *vsock)
> +				     struct virtio_vsock *vsock, gfp_t gfp)
>   {
>   	int ret, in_sg = 0, out_sg = 0;
>   	struct scatterlist **sgs;
> @@ -140,7 +140,7 @@ static int virtio_transport_send_skb(struct sk_buff *skb, struct virtqueue *vq,
>   		}
>   	}
>   
> -	ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, GFP_KERNEL);
> +	ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, gfp);
>   	/* Usually this means that there is no more space available in
>   	 * the vq
>   	 */
> @@ -178,7 +178,7 @@ virtio_transport_send_pkt_work(struct work_struct *work)
>   
>   		reply = virtio_vsock_skb_reply(skb);
>   
> -		ret = virtio_transport_send_skb(skb, vq, vsock);
> +		ret = virtio_transport_send_skb(skb, vq, vsock, GFP_KERNEL);
>   		if (ret < 0) {
>   			virtio_vsock_skb_queue_head(&vsock->send_pkt_queue, skb);
>   			break;
> @@ -221,7 +221,7 @@ static int virtio_transport_send_skb_fast_path(struct virtio_vsock *vsock, struc
>   	if (unlikely(ret == 0))
>   		return -EBUSY;
>   
> -	ret = virtio_transport_send_skb(skb, vq, vsock);
> +	ret = virtio_transport_send_skb(skb, vq, vsock, GFP_ATOMIC);
>   	if (ret == 0)
>   		virtqueue_kick(vq);
>