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

[Cong Wang]

Stephen Hemminger wrote:
> On Tue, 13 Apr 2010 09:52:47 -0700
> Jay Vosburgh <fubar@xxxxxxxxxx> wrote:
>
> > Cong Wang <amwang@xxxxxxxxxx> wrote:
> >
> > > Stephen Hemminger wrote:
> > > > On Mon, 12 Apr 2010 12:38:57 +0200
> > > > Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> > > >
> > > > > Le lundi 12 avril 2010 Ã 18:37 +0800, Cong Wang a Ãcrit :
> > > > > > 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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