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Message-ID: <4dbed8896c94a347dcb58b3a83792c52fdc1c04a.camel@sipsolutions.net>
Date: Mon, 23 Oct 2023 09:33:09 +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 Mon, 2023-10-23 at 07:08 +0000, Vincent Whitchurch wrote:
> On Fri, 2023-10-20 at 16:47 +0200, Vincent Whitchurch wrote:
> > In basic time travel mode, I sometimes see "time goes backwards" panics
> > like the one below:
> >
> > Kernel panic: time-travel: time goes backwards 161689340000492 -> 161689339869814
Ouch.
> > Call Trace:
> > panic+0x1a1/0x3d7
> > time_travel_update_time.cold+0xe9/0x133
> > timer_read+0xc1/0x100
> > ktime_get+0x10c/0x200
> > copy_process+0x1899/0x2230
> > kernel_clone+0x57/0x7a0
> > kernel_thread+0x4a/0x50
> > kthreadd+0x116/0x190
> >
> > The problem is a race between time_travel_handle_real_alarm() and
> > timer_read(). time_travel_handle_real_alarm() changes the time after
> > time_read() reads the current time but before time_travel_update_time()
> > has had a chance to add the end event.
> >
> > Fix this by doing the time read and event add atomically with respect to
> > time_travel_handle_real_alarm().
>
> Further testing resulted in hitting the BUG_ON(time_travel_time !=
> e->time) so looks like this needs some more work.
>
Yeah this is a tricky area, I fought with it for quite a while too,
seems we're not done yet ;-)
We mostly use time-travel=ext mode these days, so our system may not be
as susceptible to it? But not sure, in some cases it runs with just a
single instance, and that should be pretty much the same due to the
free-until information.
Do you have a specific workload that tends to reproduce this?
johannes
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