netdev - Re: [PATCH next v5 2/6] bonding: implement bond_poll

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Message-ID: <CANP3RGe7a9hwDB5iUapNvxVS_TD0kkmA3rWJnku=j5tUZ9v6rQ@mail.gmail.com>
Date:	Wed, 18 Feb 2015 18:26:02 -0800
From:	Maciej Żenczykowski <maze@...gle.com>
To:	Mahesh Bandewar <maheshb@...gle.com>
Cc:	Nikolay Aleksandrov <nikolay@...hat.com>,
	Jay Vosburgh <j.vosburgh@...il.com>,
	Andy Gospodarek <andy@...yhouse.net>,
	Veaceslav Falico <vfalico@...il.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH next v5 2/6] bonding: implement bond_poll_controller()

>>> Hi Mahesh,
>>> I should've explained more in my review, you cannot sleep in
>>> bond_poll_controller() so you cannot acquire rtnl like that. I was thinking
>>> more about using rcu and switching to the _rcu version of
>>> bond_for_each_slave instead.
>>>
>> That makes sense. The path that triggered this netpoll() could have
>> been holding the rtnl  itself and this would be a problem. I think
>> using the _rcu variant of the slave iterator is a good idea, my bad!
>>
> ... however we cannot use the _rcu variant either since there is the
> netpoll mutex (ni->dev_lock)!
> The fact that we are here itself means that something bad had happened
> and trying to take additional lock(s) would complicate the situation
> further.

I think you might be incorrectly assuming that we only get here on
kernel crashes,
upstream (netconsole) [may] violate[s] this assumption.

(background: internally we don't consider netconsole production
worthy, but we do
'abuse' the netpoll support framework to generate a minimal network
dump on kernel crash,
in which case if we crash or otherwise violate assumptions while
crashing, we're not any worse off...)

- Maciej
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