linux-kernel - Re: [PATCH 2/3] timekeeping: Fix potential lost pv notification of time change

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Message-ID: <20131218100807.GC19319@gmail.com>
Date:	Wed, 18 Dec 2013 11:08:07 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Vrabel <david.vrabel@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Richard Cochran <richardcochran@...il.com>,
	xen-devel@...ts.xen.org, stable <stable@...r.kernel.org>
Subject: Re: [PATCH 2/3] timekeeping: Fix potential lost pv notification of
 time change


* John Stultz <john.stultz@...aro.org> wrote:

> In 780427f0e11 (Indicate that clock was set in the pvclock
> gtod notifier), logic was added to pass a CLOCK_WAS_SET
> notification to the pvclock notifier chain.
> 
> While that patch added a action flag returned from
> accumulate_nsecs_to_secs(), it only uses the returned value
> in one location, and not in the logarithmic accumulation.
> 
> This means if a leap second triggered during the logarithmic
> accumulation (which is most likely where it would happen),
> the notification that the clock was set would not make it to
> the pv notifiers.
> 
> This patch extends the logarithmic_accumulation pass down
> that action flag so proper notification will occur.
> 
> Cc: Sasha Levin <sasha.levin@...cle.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: David Vrabel <david.vrabel@...rix.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Cc: Prarit Bhargava <prarit@...hat.com>
> Cc: Richard Cochran <richardcochran@...il.com>
> Cc: <xen-devel@...ts.xen.org>
> Cc: stable <stable@...r.kernel.org> #3.11+
> Signed-off-by: John Stultz <john.stultz@...aro.org>
> ---
>  kernel/time/timekeeping.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 6bad3d9..998ec751 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -1295,7 +1295,7 @@ static inline unsigned int accumulate_nsecs_to_secs(struct timekeeper *tk)
>   * Returns the unconsumed cycles.
>   */
>  static cycle_t logarithmic_accumulation(struct timekeeper *tk, cycle_t offset,
> -						u32 shift)
> +						u32 shift, unsigned int *action)

I have two complaints about this patch:

1)

I think the 'action' name sucks because it's too obfuscated. It's only 
ever set to TK_CLOCK_WAS_SET, so why not name it more descriptively, 
i.e. 'clock_was_set'?

2)

Secondly, the proliferation of parameters passed around I think calls 
for a helper structure which would carry the (offset, shift, 
clock_was_set) triple:

	struct acc_params {
		cycle_t		offset;
		u32		shift;
		bool		clock_was_set;
	};

And then passed down like this:

>  static cycle_t logarithmic_accumulation(struct timekeeper *tk, struct acc_params *params)

Agreed?

Thanks,

	Ingo
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