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Subject: Re: UML time-travel warning from __run_timers
From: Johannes Berg <johannes@sipsolutions.net>
Date: Sat, 02 Apr 2022 16:17:25 +0200
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To: Vincent Whitchurch <vincent.whitchurch@axis.com>
Cc: linux-um@lists.infradead.org, linux-kernel@vger.kernel.org, Anna-Maria Gleixner <anna-maria@linutronix.de>, Thomas Gleixner <tglx@linutronix.de>, Frederic Weisbecker <frederic@kernel.org>

On Sat, 2022-04-02 at 16:09 +0200, Johannes Berg wrote:
> 
> (I put a WARN_ON into get_timer_cpu_base() and get_timer_this_cpu_base()
> in the if, and it never triggered; I guess my config has something that
> creates a deferrable timer, so it didn't trigger, but I didn't check
> that now.)
> 

OK, so FWIW, I checked that now, and I have e.g. CONFIG_WQ_WATCHDOG
enabled, which makes a deferrable timer:

static void wq_watchdog_init(void)
{
        timer_setup(&wq_watchdog_timer, wq_watchdog_timer_fn, TIMER_DEFERRABLE);


but I think a bunch of other (networking) things too that end up using a
TIMER_DEFERRABLE timer, via workqueues.

But maybe this would also happen if it was used just a single time,
since the timer would run & go away, leaving base->next_expiry never to
change again? But the WQ watchdog and also neigh_managed_work() are
periodic.

johannes

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