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

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Date:	Tue, 13 Apr 2010 10:33:20 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Jay Vosburgh <fubar@...ibm.com>
Cc:	Cong Wang <amwang@...hat.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

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.



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