linux-kernel - Re: [PATCH] hrtimer, add expiry time overflow check in hrtimer

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Date:	Mon, 08 Apr 2013 10:53:08 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Prarit Bhargava <prarit@...hat.com>
CC:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCH] hrtimer, add expiry time overflow check in hrtimer_interrupt

On 04/08/2013 08:47 AM, Prarit Bhargava wrote:

> When we change the system time to a low value like this, the value of
> timekeeper->offs_real will be a negative value.
>
> It seems that the WARN occurs because an hrtimer has been started in the time
> between the releasing of the timekeeper lock and the IPI call (via a call to
> on_each_cpu) in clock_was_set() in the do_settimeofday() code.  The end result
> is that a REALTIME_CLOCK timer has been added with softexpires = expires =
> KTIME_MAX.  The hrtimer_interrupt() fires/is called and the loop at
> kernel/hrtimer.c:1289 is executed.  In this loop the code subtracts the
> clock base's offset (which was set to timekeeper->offs_real in
> do_settimeofday()) from the current hrtimer_cpu_base->expiry value (which
> was KTIME_MAX):
>
> 	KTIME_MAX - (a negative value) = overflow
>
> A simple check for an overflow can resolve this problem.  Using KTIME_MAX
> instead of the overflow value will result in the hrtimer function being run,
> and the reprogramming of the timer after that.
 >
> Signed-off-by: Prarit Bhargava <prarit@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: John Stultz <john.stultz@...aro.org>

Reviewed-by: Rik van Riel <riel@...hat.com>

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