linux-kernel - Re: [PATCH] timer: Prevent base->clk from moving backward

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Message-ID: <alpine.DEB.2.21.2007091139470.4020@somnus>
Date:   Thu, 9 Jul 2020 11:42:31 +0200 (CEST)
From:   Anna-Maria Behnsen <anna-maria@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>, Stable <stable@...nel.org>
Subject: Re: [PATCH] timer: Prevent base->clk from moving backward

Hi,

On Fri, 3 Jul 2020, Frederic Weisbecker wrote:

> When a timer is enqueued with a negative delta (ie: expiry is below
> base->clk), it gets added to the wheel as expiring now (base->clk).
> 
> Yet the value that gets stored in base->next_expiry, while calling
> trigger_dyntick_cpu(), is the initial timer->expires value. The
> resulting state becomes:
> 
> 	base->next_expiry < base->clk
> 
> On the next timer enqueue, forward_timer_base() may accidentally
> rewind base->clk. As a possible outcome, timers may expire way too
> early, the worst case being that the highest wheel levels get spuriously
> processed again.
> 
> To prevent from that, make sure that base->next_expiry doesn't get below
> base->clk.
> 
> Fixes: a683f390b93f ("timers: Forward the wheel clock whenever possible")
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Anna-Maria Gleixner <anna-maria@...utronix.de>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> ---

Reviewed-by: Anna-Maria Behnsen <anna-maria@...utronix.de>

Thanks,

	Anna-Maria