linux-kernel - Re: UML time-travel warning from __run

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Date:   Mon, 04 Apr 2022 10:32:46 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Johannes Berg <johannes@...solutions.net>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     linux-um@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: UML time-travel warning from __run_timers

On Mon, Apr 04 2022 at 09:02, Johannes Berg wrote:
> On Sun, 2022-04-03 at 21:51 +0200, Thomas Gleixner wrote:
>> but that's fine and it is overwritten by every timer which is inserted
>> to expire before that. So that's not an issue as the prandom timer is
>> firing and rearmed.
>
> No, as I said before, there's never any timer with base 1 (BASE_DEF) in
> the config we have. The prandom timer is not TIMER_DEFERRABLE (it
> probably could be, but it's not now). There's no deferrable timer at
> all. Once there is at least one, the warning goes away.

Groan. I overlooked the deferrable part. Yes, you are right. next_expiry
of the deferrable base is stale when there is no timer queued up to the
point where base->clk reaches the initial next_expiry value. So the
check is bogus.

Thanks,

        tglx
---
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1724,9 +1724,8 @@ static inline void __run_timers(struct t
 		/*
 		 * The only possible reason for not finding any expired
 		 * timer at this clk is that all matching timers have been
-		 * dequeued.
+		 * dequeued or no timer has been ever queued.
 		 */
-		WARN_ON_ONCE(!levels && !base->next_expiry_recalc);
 		base->clk++;
 		base->next_expiry = __next_timer_interrupt(base);