Subject: Re: [PATCH] bonding: remove sysfs before removing devices

From: Veaceslav Falico <[email-masked]>
Date: Sat, 6 Apr 2013 03:49:24 +0200

On Sat, Apr 06, 2013 at 01:29:48AM +0200, Nikolay Aleksandrov wrote:
>On 04/06/2013 01:21 AM, Veaceslav Falico wrote:
>> On Sat, Apr 06, 2013 at 12:15:11AM +0200, Nikolay Aleksandrov wrote:
>>> Hi,
>>> Sorry for the late reply but I was travelling this week. In my
>>> opinion this
>>> fix is wrong because in bond_uninit() (called by rtnl_link_unregister)
>>> you have:
>>> list_del(&bond->bond_list);
>>> which is linked in the bond_net dev_list which is freed by
>>> unregister_pernet_subsys.
>>
>> Yep, you're right, I've hit it recently with the patch applied, and now
>> working on it. However, I still think that the idea of the patch is
>> correct
>> - i.e. to first disable sysfs (especially bonding_masters) and only
>>  afterwards to start removing everything else. Or, obviously, to finally
>> add normal locking to sysfs functions.
>>
>> Anyway, this corruption is really rare, so I'll wait for your fix next
>> week.
>>
>Well, there's no need for that because you'll have to iterate over all
>"net"s to do it properly. Since we already have code that does it
>(unregister_pernet_subsys), my fix is to kill off any "left" bond
>devices in bond_net_exit() _after_ destroying the bonding_masters
>sysfs entry for that net. This way we preserve the code structure and avoid
>duplicating a loop over all nets.

Yep, that was one approach that I wanted to do, however I didn't like the
idea to duplicate the device destruction - i.e. the rtnl_link_unregister()
already does that, and to re-delete them after sysfs gets out of the way
feeled wrong. However, I've also missed the net->dev_list part, so seems
like both approaches have drawbacks... Or did I miss something?

>This is the fix we discussed a week ago.

Not with me :). I didn't know of this bug by that time...

>I'd be happy to hear any comments on it before posting :-)
>
>Of course stopping the whole bonding sysfs handling is also an alternative.

I think it'd be the best way to do that.

1) We can't remove the net_ns before removing the devices cause they depend
on it (and I think it's quite a hack anyway now that I'm aware of dev_list).
2) We can't re-remove devices after sysfs deactivation, it's also a hack,
cause rtnl does the same thing.
3) We can't hold rtnl_mutex to do both rtnl and net_ns removal cause we can
deadlock in sysfs code (when it gets removed).
4) We can't remove the b_masters before all the logic cause it'll duplicate
the code for unregister_pernet_operations(), and will also look like a
hack (with looping through net_ns).

Out of all these, #2 (your option) looks the best - the least intrusive and
the easiest to read. However, I still think that it's the lesser evil, and
there must exist a way to do it correctly (like with #3, and avoid the
deadlock by restart_syscall() technique in bond_store_bonds() - however I
don't really know if it'll work.).

Anyway, I'd really like your feedback on these thoughts, and if nothing
better comes up - your patch :).

Thank you!