Re: [Patch net-next v3] netpoll: fix a rtnl lock assertion failure

[Eric Dumazet <eric.dumazet@gmail.com>]

On Mon, 2013-01-14 at 18:43 +0800, Cong Wang wrote:
> From: Cong Wang <amwang@redhat.com>
> 
> This patch fixes the following warning:
> 
> [   72.013864] RTNL: assertion failed at net/core/dev.c (4955)
> [   72.017758] Pid: 668, comm: netpoll-prep-v6 Not tainted 3.8.0-rc1+ #474
> [   72.019582] Call Trace:
> [   72.020295]  [<ffffffff8176653d>] netdev_master_upper_dev_get+0x35/0x58
> [   72.022545]  [<ffffffff81784edd>] netpoll_setup+0x61/0x340
> [   72.024846]  [<ffffffff815d837e>] store_enabled+0x82/0xc3
> [   72.027466]  [<ffffffff815d7e51>] netconsole_target_attr_store+0x35/0x37
> [   72.029348]  [<ffffffff811c3479>] configfs_write_file+0xe2/0x10c
> [   72.030959]  [<ffffffff8115d239>] vfs_write+0xaf/0xf6
> [   72.032359]  [<ffffffff81978a05>] ? sysret_check+0x22/0x5d
> [   72.033824]  [<ffffffff8115d453>] sys_write+0x5c/0x84
> [   72.035328]  [<ffffffff819789d9>] system_call_fastpath+0x16/0x1b
> 
> Just hold RCU read lock and call netdev_master_upper_dev_get_rcu(),
> as suggested by Jiri.
> 
> Cc: Jiri Pirko <jiri@resnulli.us>
> Cc: David S. Miller <davem@davemloft.net>
> Signed-off-by: Cong Wang <amwang@redhat.com>
> 
> ---
> diff --git a/net/core/netpoll.c b/net/core/netpoll.c
> index 9f05067..5d1f856 100644
> --- a/net/core/netpoll.c
> +++ b/net/core/netpoll.c
> @@ -1055,11 +1055,14 @@ int netpoll_setup(struct netpoll *np)
>  		return -ENODEV;
>  	}
>  
> -	if (netdev_master_upper_dev_get(ndev)) {
> +	rcu_read_lock();
> +	if (netdev_master_upper_dev_get_rcu(ndev)) {
> +		rcu_read_unlock();
>  		np_err(np, "%s is a slave device, aborting\n", np->dev_name);
>  		err = -EBUSY;
>  		goto put;
>  	}
> +	rcu_read_unlock();
>  
>  	if (!netif_running(ndev)) {
>  		unsigned long atmost, atleast;
> --

Strange, why dont we call netdev_master_upper_dev_get() instead ?