netdev - Re: [Patch] bonding: fix netpoll in active-backup mode

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Message-ID: <4D75E83C.5030609@redhat.com>
Date:	Tue, 08 Mar 2011 16:26:36 +0800
From:	Cong Wang <amwang@...hat.com>
To:	Neil Horman <nhorman@...driver.com>
CC:	linux-kernel@...r.kernel.org, Jay Vosburgh <fubar@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"John W. Linville" <linville@...driver.com>,
	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Subject: Re: [Patch] bonding: fix netpoll in active-backup mode

于 2011年03月08日 12:15, Cong Wang 写道:
> 于 2011年03月08日 02:50, Neil Horman 写道:
>> On Mon, Mar 07, 2011 at 10:11:50PM +0800, Amerigo Wang wrote:
>>> netconsole doesn't work in active-backup mode, because we don't do anything
>>> for nic failover in active-backup mode. This patch fixes the problem by:
>>>
>>> 1) make slave_enable_netpoll() and slave_disable_netpoll() callable in softirq
>>> context, that is, moving code after synchronize_rcu_bh() into call_rcu_bh()
>>> callback function, teaching kzalloc() to use GFP_ATOMIC.
>>>
>>> 2) disable netpoll on old slave and enable netpoll on the new slave.
>>>
>>> Tested by ifdown the current active slave and ifup it again for several times,
>>> netconsole works well.
>>>
>>> Signed-off-by: WANG Cong<amwang@...hat.com>
>>>
>> I may be missing soething but this seems way over-complicated to me. I presume
>> the problem is that in active backup mode a failover results in the new active
>> slave not having netpoll setup on it? If thats the case, why not just setup
>> netpoll on all slaves when ndo_netpoll_setup is called on the bonding interface?
>> I don't see anything immeidately catastrophic that would happen as a result.
>
>
> But we still need to clean up the netpoll on the failing slave, which still
> needs to call slave_disable_netpoll() in monitor code, I see no big differences
> with the solution I take.
>
>
>> And then you wouldn't have to worry about disabling/enabling anything on a
>> failover (or during a panic for that matter). As for the rcu bits? Why are
>> they needed? One would presume that wouldn't (or at least shouldn't) be able to
>> teardown our netpoll setup until such time as all the pending frames for that
>> netpoll client have been transmitted. If we're not blocknig on that RCU isn't
>> really going to help. Seems like the proper fix is take a reference to the
>> appropriate npinfo struct in netpoll_send_skb, and drop it from the skbs
>> destructor or some such.
>
> I saw a "scheduling while in atomic" warning without touching the rcu bits.
>

Hmm, I was wrong, this warning is misleading, I think the root cause is that
I call slave_disable_netpoll() with write_lock_bh() held...

Will update the patch soon...
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