linux-kernel - Re: [Regression] Negative time on Acer Ferrari One with current -git

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Date:	Sun, 23 May 2010 15:21:02 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	John Stultz <johnstul@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [Regression] Negative time on Acer Ferrari One with current -git

On Sunday 23 May 2010, Thomas Gleixner wrote:
> On Sat, 22 May 2010, Rafael J. Wysocki wrote:
> > After reverting commit 64ce4c2f above things work again.
> > 
> > To be precise, I reverted both commit 64ce4c2f and commit 6a867a3 (time: 
> > Remove xtime_cache), but since the symptoms continued to apprear after
> > reverting the latter alone, it's quite clear that commit 64ce4c2f breaks things
> > on this box.
> 
> I assume the cmos clock of this machine is not on UTC, right ?

Yup.

> Does the patch below fix the issue ?

Yes, it does, thanks!

> @John: Can you please check the other users of timespec_add_safe() in
> timekeeping as well ?
> 
> Thanks,
> 
> 	tglx
> 
> ---------->
> Subject: timekeeping: Fix timezone update
> From: Thomas Gleixner <tglx@...utronix.de>
> Date: Sun, 23 May 2010 08:14:45 +0200
> 
> commit 64ce4c2f (time: Clean up warp_clock()) breaks the timezone
> update in a very subtle way. To avoid the direct access to timekeeping
> internals it adds the timezone delta to the current time with
> timespec_add_safe(). This works nicely when the timezone delta is > 0.
> If timezone delta is < 0 then the wrap check in timespec_add_safe()
> triggers and timespec_add_safe() returns TIME_MAX and screws up
> timekeeping completely. 
> 
> This is not surprising as the comment above timespec_add_safe() says:
>     It's assumed that both values are valid (>= 0)
> 
> The function was created to avoid overflow issues when adding the
> select() timeout to current time, where the above applies.
> 
> Add the timezone seconds adjustment directly.
> 
> Reported-by: Rafael J. Wysocki <rjw@...k.pl>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  kernel/time.c |    8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> Index: linux-2.6/kernel/time.c
> ===================================================================
> --- linux-2.6.orig/kernel/time.c
> +++ linux-2.6/kernel/time.c
> @@ -132,10 +132,10 @@ SYSCALL_DEFINE2(gettimeofday, struct tim
>   */
>  static inline void warp_clock(void)
>  {
> -	struct timespec delta, adjust;
> -	delta.tv_sec = sys_tz.tz_minuteswest * 60;
> -	delta.tv_nsec = 0;
> -	adjust = timespec_add_safe(current_kernel_time(), delta);
> +	struct timespec adjust;
> +
> +	adjust = current_kernel_time();
> +	adjust.tv_sec += sys_tz.tz_minuteswest * 60;
>  	do_settimeofday(&adjust);
>  }

Rafael
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