netdev - Re: [PATCH net] netvsc: fix net poll mode

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Date:   Fri, 12 May 2017 14:15:26 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     davem@...emloft.net, netdev@...r.kernel.org,
        Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: [PATCH net] netvsc: fix net poll mode

On Fri, 2017-05-12 at 13:03 -0700, Stephen Hemminger wrote:
> The ndo_poll_controller function needs to schedule NAPI to pick
> up arriving packets and send completions. Otherwise no data
> will ever be received.
> 
> Signed-off-by: Stephen Hemminger <sthemmin@...rosoft.com>
> ---
>  drivers/net/hyperv/netvsc_drv.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/hyperv/netvsc_drv.c b/drivers/net/hyperv/netvsc_drv.c
> index 4421a6d00375..e487ccea251c 100644
> --- a/drivers/net/hyperv/netvsc_drv.c
> +++ b/drivers/net/hyperv/netvsc_drv.c
> @@ -1158,11 +1158,20 @@ netvsc_get_rxnfc(struct net_device *dev, struct ethtool_rxnfc *info,
>  }
>  
>  #ifdef CONFIG_NET_POLL_CONTROLLER
> -static void netvsc_poll_controller(struct net_device *net)
> +static void netvsc_poll_controller(struct net_device *dev)
>  {
> -	/* As netvsc_start_xmit() works synchronous we don't have to
> -	 * trigger anything here.
> -	 */
> +	struct net_device_context *ndc = netdev_priv(dev);
> +	struct netvsc_device *ndev = rtnl_dereference(ndc->nvdev);


rtnl_dereference() can not possibly be used in an ndo_poll_controller()

You would certainly trigger a lockdep issue here if you compile a kernel
with enough debugging support.

CONFIG_PROVE_RCU=y
CONFIG_PROVE_RCU_REPEATEDLY=y
CONFIG_SPARSE_RCU_POINTER=y