Subject: Re: [PATCH] neighbour.c: Avoid GC directly after state change

From: Ulf Samuelsson <[email-masked]>
Date: Fri, 10 Apr 2015 10:26:21 +0200

On 03/12/2015 07:26 PM, David Miller wrote:
> I hate changes like this.
>
> By making this a Kconfig options it cannot be dynamic, and every
> distribution is going to have to scratch their head and decide
> what to set this to.
>
> That's seriously suboptimal.
>
> If you want to change behavior based upon whether userspace is
> managing the damn table, make it so the user doing so has to
> ask for the new behavior at _RUN TIME_ via the netlink interface
> or similar.
>
> Picking the guard time itself at compile time is also undesirable.
>
> And you don't even want a damn timer, what you want is for the
> state of the entry to be frozen and for the user to "release"
> it by either adjusting the state to something else or marking
> in some other way to allow it to be unfrozen and released again.
>
> Why put it to chance with some timeout?  Make things explicit.

The desired functionality is that if communication stops,
you want to send out ARP probes, before the entry is deleted.

The current (pseudo) code of the neigh timer is:

     if (state & NUD_REACHABLE) {
         if (now <= "confirmed + "reachable_time")) {
                     ... /* We are OK */
         } else if (now < "used" + DELAY_PROBE_TIME) {    /* Never 
happens */
                     state = NUD_DELAY;
         } else {
             state = NUD_STALE;
             notify = 1;
         }

We never see the state beeing changed from REACHABLE to DELAY,
so the probes are not beeing sent out, instead you always go
from REACHABLE to STALE.

DELAY_PROBE_TIME is set to (5 x HZ) and "used"
seems to be only set by the periodic_work routine
when the neigh entry is in STALE state, and then it is too late.
It is also set by "arp_find" which is used by "broken" devices.

In practice, the second condition: "(now < "used" + DELAY_PROBE_TIME)" 
is never used.
What is the intention of this test?

By adding a new test + parameter, we would get the desired functionality,
and no need to listen for notifications or doing ARP state updates from 
applications.

         if (now <= "confirmed + "reachable_time")) {
                     ... /* We are OK */
+        else if (now <= "confirmed + "reprobe_time")) {
+                   state <= NUD_DELAY;
         } else if (now < "used" + DELAY_PROBE_TIME))) {    /* Never 
happens */
                     state <= NUD_DELAY;
         } else {
             state = NUD_STALE;
             notify = 1;
         }

This way the entry would remain in REACHABLE while normal communication 
occurs,
then it would enter DELAY state to probe, and if that fails, it goes to 
STALE state.

Alternatively, we just change the second test:
         if (now <= "confirmed + "reachable_time")) {
                     ... /* We are OK */
-        } else if (now < "used" + DELAY_PROBE_TIME))) {    /* Never 
happens */
+       } else if (now < "confirmed" + DELAY_PROBE_TIME))) {
                     state <= NUD_DELAY;
         } else {
             state = NUD_STALE;
             notify = 1;
         }

The  DELAY_PROBE_TIME, should preferrably be a kernel Kconfig parameter.

Best Regards,
Ulf Samuelsson

> I'm not applying this patch.
>
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