linux-kernel - Re: [PATCH] um: time-travel: fix time going backwards

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Date:   Wed, 25 Oct 2023 21:51:45 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Vincent Whitchurch <Vincent.Whitchurch@...s.com>,
        "anton.ivanov@...bridgegreys.com" <anton.ivanov@...bridgegreys.com>,
        "richard@....at" <richard@....at>
Cc:     "linux-um@...ts.infradead.org" <linux-um@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel <kernel@...s.com>
Subject: Re: [PATCH] um: time-travel: fix time going backwards

On Wed, 2023-10-25 at 11:55 +0000, Vincent Whitchurch wrote:
> On Mon, 2023-10-23 at 09:33 +0200, Johannes Berg wrote:
> > Do you have a specific workload that tends to reproduce this?
> 
> I've been seeing it when running roadtest, but it's easily reproducible
> without that by using the attached config and the following program as
> init.
> 
>   cp repro.config .config
>   make ARCH=um olddefconfig all
>   gcc -Wall -static -o repro repro.c
>   ./linux time-travel init=$PWD/repro rootfstype=hostfs
> 
> With the above commands I usually see the panic in a few seconds.  This
> is on an unmodified v6.6-rc7.
> 

Yes, I can reproduce it with your test, thanks. I'm on 6.4-rc still for
$reasons (6.5 we skipped during vacations, and 6.6 EEVDF scheduler broke
everything for hwsim tests in hostap), but this code didn't really
change anyway.

I'll poke at this.

(And I'm still amazed that anyone other than me even uses this stuff :P)

johannes