Subject: Re: BUG: bonding module can only be loaded once

From: Arnd Bergmann <[email-masked]>
Date: Tue, 9 Jun 2009 14:06:44 +0200

On Tuesday 09 June 2009, Stephen Hemminger wrote:
> In order to create multiple bonding dynamically, it is common practice to
> load the bonding module multiple times.  This got broken in recent kernels
> 2.6.29 and later.
> 
> Doing the following will OOPS:
>   modprobe -o bond0 bonding
>   modprobe -o bond1 bonding
> 
> 2.6.29 actually OOPS on error handling, but that is fixed in 2.6.30.
> But 2.6.30 still has the regression (caused by sysfs).
> 
> This regression was introduced by changes to sysfs and proc that
> made duplicate insertion a problem.

Well, I guess it's more like the changes just made it obvious that
it's wrong to do this. Registering the same entries in procfs or sysfs 
means that the user will only be able to talk to one of the two
bonding drivers through these interfaces. 

The log messages you quoted are not actually Oops but rather WARNING,
which is (in this case) not fatal at all, just an indication that the
root user did something he should not have:

> [  134.012578] WARNING: at fs/proc/generic.c:590 proc_register+0x154/0x191()
> [  134.012583] proc_dir_entry 'net/bonding' already registered

> [  134.014516] WARNING: at fs/sysfs/dir.c:487 sysfs_add_one+0xcc/0xe4()
> [  134.014521] sysfs: cannot create duplicate filename '/class/net/bonding_masters'

The bonding driver could work around this by checking if the directories
already exist before registering them. One can also add rtnl_link_ops to
the driver for dynamically adding more interfaces.

If you combine the two, you can even print a helpful message like 'please
use "ip link add type bonding" instead of "modprobe -o bond0 bonding"'.

In the mean time, you could probably work around this by ignoring the error
condition (see below), but I would suspect that there may be more problems
with the concept of just loading the module again. The best advice to
users is probably to configure the maximum number of bonding devices they
might need with the max_bonds= module parameter (if I understand that
parameter correctly.

	Arnd <><

--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -5203,7 +5203,7 @@ static int __init bonding_init(void)
 
 	res = bond_create_sysfs();
 	if (res)
-		goto err;
+		pr_info("Loading bonding module without sysfs interface\n");
 
 	register_netdevice_notifier(&bond_netdev_notifier);
 	register_inetaddr_notifier(&bond_inetaddr_notifier);