Re: [Patch net-next v2] netpoll: fix a rtnl lock assertion failure

[Jiri Pirko <jiri@resnulli.us>]

Mon, Jan 14, 2013 at 11:24:14AM CET, amwang@redhat.com 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..6d56b57 100644
>--- a/net/core/netpoll.c
>+++ b/net/core/netpoll.c
>@@ -1055,11 +1055,15 @@ int netpoll_setup(struct netpoll *np)
> 		return -ENODEV;
> 	}
> 
>-	if (netdev_master_upper_dev_get(ndev)) {
>+	rcu_read_lock();
>+	/* We permit devices with VLAN attached to */

I think that this comment might be misleading. Master upper is not set
not only for VLAN but for macvlan (and in future possibly for others) as well.	

Apart from this, the patch looks good to me.

>+	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;