netdev - Re: Regression in IPv6 autoconf, maybe "ipv6/addrconf: fix timing bug in tempaddr regen"

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Message-ID: <s5n6r540-o061-3n7o-28qo-16r6s4354ns0@vanv.qr>
Date: Thu, 1 Jun 2023 13:51:52 +0200 (CEST)
From: Jan Engelhardt <jengelh@...i.de>
To: Sam Edwards <cfsworks@...il.com>
cc: Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
    "David S. Miller" <davem@...emloft.net>
Subject: Re: Regression in IPv6 autoconf, maybe "ipv6/addrconf: fix timing
 bug in tempaddr regen"

On Wednesday 2023-05-31 18:31, Sam Edwards wrote:
> On 5/31/23 03:20, Jan Engelhardt wrote:
>> Greetings.
>> 
>> I am observing that between kernel 5.19 and 6.0, a change was introduced
>> that makes the system just stop generating IPv6 Privacy Addresses after
>> some time.
>
> I'd been encountering this exact problem very reliably since at least the
> early 4.x days, which was my motivation for authoring this patch (which had
> fully fixed the problem for me). [...] So imagine my surprise to learn that
> [...] but that my patch is evidently making the problem happen *more*
> frequently in your case!

Interesting. So then, I will abandon the hypothesis of a regression
(and the bisect so far) and instead focus on finding the deeper
cause. If you say it has been a problem since 4.x, then my
observation that it seemingly worked in 5.19 doesn't hold water.
Also, more frequent occurrence with 6.x is decidedly a good thing for
me debugging this.

With some classic try-it-and-see printk'ing, my current of insight is
that this branch was taken when things start to go wrong:

        age = (now - tmp_tstamp + ADDRCONF_TIMER_FUZZ_MINUS) / HZ;
        if (cfg.preferred_lft <= regen_advance + age) {
                in6_ifa_put(ifp);
                in6_dev_put(idev);
                ret = -1;
                goto out;
        }

Now, I just need to dump the timer numbers to see how
this came together, and correlate with the RAs from the CPE.

> Since we're having some pretty extreme Works On My Machine Syndrome here, we
> should really figure out what's different in your case from mine. What arch are
> you building these kernels for? Could you share the defconfig? Are you able to
> see this on multiple machines/networks or have you only tried the one? Is this
> machine real hardware or a VM?

Fujitsu U7311 x86_64, http://inai.de/files/ipv6-config.txt
Tried only one so far.