linux-kernel - Re: [RFC PATCH 01/10] timer: Prevent base->clk from moving backward

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Date:   Thu, 02 Jul 2020 11:48:23 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [RFC PATCH 01/10] timer: Prevent base->clk from moving backward

Frederic Weisbecker <frederic@...nel.org> writes:
> 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.

Bah. Nice catch.

> To prevent from that, make sure that base->next_expiry doesn't get below
> base->clk.

That wants a Fixes: tag and a CC stable.

Thanks,

        tglx