netdev - Re: [Bonding-devel] [v3 Patch 2/3] bridge: make bridge support netpoll

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Date:	Wed, 14 Apr 2010 16:16:09 +0800
From:	Cong Wang <amwang@...hat.com>
To:	Stephen Hemminger <shemminger@...ux-foundation.org>
CC:	Jay Vosburgh <fubar@...ibm.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Neil Horman <nhorman@...driver.com>, netdev@...r.kernel.org,
	Andy Gospodarek <gospo@...hat.com>,
	bridge@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	bonding-devel@...ts.sourceforge.net,
	Jeff Moyer <jmoyer@...hat.com>, Matt Mackall <mpm@...enic.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [Bonding-devel] [v3 Patch 2/3] bridge: make bridge support netpoll

Stephen Hemminger wrote:
> On Tue, 13 Apr 2010 09:52:47 -0700
> Jay Vosburgh <fubar@...ibm.com> wrote:
> 
>> Cong Wang <amwang@...hat.com> wrote:
>>
>>> Stephen Hemminger wrote:
>>>> On Mon, 12 Apr 2010 12:38:57 +0200
>>>> Eric Dumazet <eric.dumazet@...il.com> wrote:
>>>>
>>>>> Le lundi 12 avril 2010 à 18:37 +0800, Cong Wang a écrit :
>>>>>> Stephen Hemminger wrote:
>>>>>>> There is no protection on dev->priv_flags for SMP access.
>>>>>>> It would better bit value in dev->state if you are using it as control flag.
>>>>>>>
>>>>>>> Then you could use 
>>>>>>> 			if (unlikely(test_and_clear_bit(__IN_NETPOLL, &skb->dev->state)))
>>>>>>> 				netpoll_send_skb(...)
>>>>>>>
>>>>>>>
>>>>>> Hmm, I think we can't use ->state here, it is not for this kind of purpose,
>>>>>> according to its comments.
>>>>>>
>>>>>> Also, I find other usages of IFF_XXX flags of ->priv_flags are also using
>>>>>> &, | to set or clear the flags. So there must be some other things preventing
>>>>>> the race...
>>>>> Yes, its RTNL that protects priv_flags changes, hopefully...
>>>>>
>>>> The patch was not protecting priv_flags with RTNL.
>>>> For example..
>>>>
>>>>
>>>> @@ -308,7 +312,9 @@ static void netpoll_send_skb(struct netp
>>>>  		     tries > 0; --tries) {
>>>>  			if (__netif_tx_trylock(txq)) {
>>>>  				if (!netif_tx_queue_stopped(txq)) {
>>>> +					dev->priv_flags |= IFF_IN_NETPOLL;
>>>>  					status = ops->ndo_start_xmit(skb, dev);
>>>> +					dev->priv_flags &= ~IFF_IN_NETPOLL;
>>>>  					if (status == NETDEV_TX_OK)
>>>>  						txq_trans_update(txq);
>>> Hmm, but I checked the bonding case (IFF_BONDING), it doesn't
>>> hold rtnl_lock. Strange.
>> 	I looked, and there are a couple of cases in bonding that don't
>> have RTNL for adjusting priv_flags (in bond_ab_arp_probe when no slaves
>> are up, and a couple of cases in 802.3ad).  I think the solution there
>> is to move bonding away from priv_flags for some of this (e.g., convert
>> bonding to use a frame hook like bridge and macvlan, and greatly
>> simplify skb_bond_should_drop), but that's a separate topic.
>>
>> 	The majority of the cases, however, do hold RTNL.  Bonding
>> generally doesn't have to acquire RTNL itself, since whatever called
>> into bonding is holding it already.  For example, the slave add and
>> remove paths (bond_enslave, bond_release) are called either via sysfs or
>> ioctl, both of which acquire RTNL.  All of the set and clear operations
>> for IFF_BONDING fall into this category; look at bonding_store_slaves
>> for an example.
>>
>> 	Bonding does acquire RTNL itself when performing failovers,
>> e.g., bond_mii_monitor holds RTNL prior to calling bond_miimon_commit,
>> which will change priv_flags.
>>
> 
> All this was related to netpoll. And netpoll processing often needs to occur
> in hard IRQ context. Therefor netpoll stuff and RTNL (which is a mutex),
> really don't mix well.  Keep RTNL for what it was meant for network
> reconfiguration. Don't turn it into a network special BKL.
> 

Hmm, I think for my patch, holding RTNL lock is not necessary,
because there're no other call pathes to change IFF_IN_NETPOLL bit,
which is unlike bonding or bridge cases where sysfs/ioctl is provided
to change it.

The only chance to change IFF_IN_NETPOLL is in netpoll_send_skb()
which can't be called simultaneously because there are other locks
protecting it.

Or am I still missing something?

Thanks.
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