netdev - Re: [PATCH net-next] Better handling of transition to NUD

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Message-ID: <1432138225.3278.9.camel@stressinduktion.org>
Date:	Wed, 20 May 2015 18:10:25 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Lorenzo Colitti <lorenzo@...gle.com>
Cc:	Erik Kline <ek@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next] Better handling of transition to NUD_PROBE
 state

On Mi, 2015-05-20 at 12:42 +0900, Lorenzo Colitti wrote:
> On Mon, May 18, 2015 at 7:44 PM, Erik Kline <ek@...gle.com> wrote:
> > [1] When entering NUD_PROBE state via neigh_update(), perhaps received
> >     from userspace, correctly (re)initialize the probes count to zero.
> >
> >     This is useful for forcing revalidation of a neighbor (for example
> >     if the host is attempting to do DNA [IPv4 4436, IPv6 6059]).
> >
> > [2] Notify listeners when a neighbor goes into NUD_PROBE state.
> >
> >     By sending notifications on entry to NUD_PROBE state listeners get
> >     more timely warnings of imminent connectivity issues.
> >
> >     The current notifications on entry to NUD_STALE have somewhat
> >     limited usefulness: NUD_STALE is a perfectly normal state, as is
> >     NUD_DELAY, whereas notifications on entry to NUD_FAILURE come after
> >     a neighbor reachability problem has been confirmed (typically after
> >     three probes).
> >
> > Signed-off-by: Erik Kline <ek@...gle.com>
> > ---
> >  net/core/neighbour.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> > index 3de6542..3a74df7 100644
> > --- a/net/core/neighbour.c
> > +++ b/net/core/neighbour.c
> > @@ -913,6 +913,7 @@ static void neigh_timer_handler(unsigned long arg)
> >                         neigh->nud_state = NUD_PROBE;
> >                         neigh->updated = jiffies;
> >                         atomic_set(&neigh->probes, 0);
> > +                       notify = 1;
> 
> +1. Currently, the code notifies when going from REACHABLE into STALE,
> which is not necessarily something userspace might want to know about
> (all it means is "we haven't sent any packets to this neighbour
> recently"), but it doesn't notify when going into PROBE, which is a
> more important event (it means "we've been sending this neighbour
> packets for (by default) 5 seconds and we still haven't found out if
> it's stilll there, so we're probing it").
> 
> >                         next = now + NEIGH_VAR(neigh->parms, RETRANS_TIME);
> >                 }
> >         } else {
> > @@ -1144,6 +1145,8 @@ int neigh_update(struct neighbour *neigh, const u8 *lladdr, u8 new,
> >
> >         if (new != old) {
> >                 neigh_del_timer(neigh);
> > +               if (new & NUD_PROBE)
> > +                       atomic_set(&neigh->probes, 0);
> 
> +1. The normal code path (from STALE to DELAY to PROBE) obviously
> already sets the probes to 0. Userspace can put a neighbour into
> NUD_PROBE to cause the kernel to probe it, but this only works (by
> default) three times because the probe counter is never reset to 0. So
> the first three times, the neighbour goes from (say) STALE to PROBE
> and then back to REACHABLE (good), but then the fourth time, the
> neighbour goes from STALE to PROBE and then immediately to FAILED.
> 
> >                 if (new & NUD_IN_TIMER)
> >                         neigh_add_timer(neigh, (jiffies +
> >                                                 ((new & NUD_REACHABLE) ?
> > --
> > 2.2.0.rc0.207.ga3a616c
> >
> 
> Acked-By: Lorenzo Colitti <lorenzo@...gle.com>

I agree with Lorenzo, these changes look fine.

Acked-by: Hannes Frederic Sowa <hannes@...essinduktion.org>


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