ACK w/cmt: [PATCH Xenial SRU] clocksource: Allow unregistering the watchdog

[Chris J Arges]

Clean cherry-pick, already in 4.6.

Were you able to crashdump a Xenial hyper-v guest without this patch? It worked
for me last time I tried a few weeks ago.

--chris

On Fri, Jun 03, 2016 at 08:38:35PM -0600, Tim Gardner wrote:
> From: Vitaly Kuznetsov <vkuznets at redhat.com>
> 
> BugLink: http://bugs.launchpad.net/bugs/1588965
> 
> Hyper-V vmbus module registers TSC page clocksource when loaded. This is
> the clocksource with the highest rating and thus it becomes the watchdog
> making unloading of the vmbus module impossible.
> Separate clocksource_select_watchdog() from clocksource_enqueue_watchdog()
> and use it on clocksource register/rating change/unregister.
> 
> After all, lobotomized monkeys may need some love too.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets at redhat.com>
> Cc: John Stultz <john.stultz at linaro.org>
> Cc: Dexuan Cui <decui at microsoft.com>
> Cc: K. Y. Srinivasan <kys at microsoft.com>
> Link: http://lkml.kernel.org/r/1453483913-25672-1-git-send-email-vkuznets@redhat.com
> Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
> (cherry picked from commit bbf66d897adf2bb0c310db96c97e8db6369f39e1)
> Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
> ---
>  kernel/time/clocksource.c | 52 ++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 42 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> index 1347882..b98810d 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -323,13 +323,42 @@ static void clocksource_enqueue_watchdog(struct clocksource *cs)
>  		/* cs is a watchdog. */
>  		if (cs->flags & CLOCK_SOURCE_IS_CONTINUOUS)
>  			cs->flags |= CLOCK_SOURCE_VALID_FOR_HRES;
> +	}
> +	spin_unlock_irqrestore(&watchdog_lock, flags);
> +}
> +
> +static void clocksource_select_watchdog(bool fallback)
> +{
> +	struct clocksource *cs, *old_wd;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&watchdog_lock, flags);
> +	/* save current watchdog */
> +	old_wd = watchdog;
> +	if (fallback)
> +		watchdog = NULL;
> +
> +	list_for_each_entry(cs, &clocksource_list, list) {
> +		/* cs is a clocksource to be watched. */
> +		if (cs->flags & CLOCK_SOURCE_MUST_VERIFY)
> +			continue;
> +
> +		/* Skip current if we were requested for a fallback. */
> +		if (fallback && cs == old_wd)
> +			continue;
> +
>  		/* Pick the best watchdog. */
> -		if (!watchdog || cs->rating > watchdog->rating) {
> +		if (!watchdog || cs->rating > watchdog->rating)
>  			watchdog = cs;
> -			/* Reset watchdog cycles */
> -			clocksource_reset_watchdog();
> -		}
>  	}
> +	/* If we failed to find a fallback restore the old one. */
> +	if (!watchdog)
> +		watchdog = old_wd;
> +
> +	/* If we changed the watchdog we need to reset cycles. */
> +	if (watchdog != old_wd)
> +		clocksource_reset_watchdog();
> +
>  	/* Check if the watchdog timer needs to be started. */
>  	clocksource_start_watchdog();
>  	spin_unlock_irqrestore(&watchdog_lock, flags);
> @@ -404,6 +433,7 @@ static void clocksource_enqueue_watchdog(struct clocksource *cs)
>  		cs->flags |= CLOCK_SOURCE_VALID_FOR_HRES;
>  }
>  
> +static void clocksource_select_watchdog(bool fallback) { }
>  static inline void clocksource_dequeue_watchdog(struct clocksource *cs) { }
>  static inline void clocksource_resume_watchdog(void) { }
>  static inline int __clocksource_watchdog_kthread(void) { return 0; }
> @@ -736,6 +766,7 @@ int __clocksource_register_scale(struct clocksource *cs, u32 scale, u32 freq)
>  	clocksource_enqueue(cs);
>  	clocksource_enqueue_watchdog(cs);
>  	clocksource_select();
> +	clocksource_select_watchdog(false);
>  	mutex_unlock(&clocksource_mutex);
>  	return 0;
>  }
> @@ -758,6 +789,7 @@ void clocksource_change_rating(struct clocksource *cs, int rating)
>  	mutex_lock(&clocksource_mutex);
>  	__clocksource_change_rating(cs, rating);
>  	clocksource_select();
> +	clocksource_select_watchdog(false);
>  	mutex_unlock(&clocksource_mutex);
>  }
>  EXPORT_SYMBOL(clocksource_change_rating);
> @@ -767,12 +799,12 @@ EXPORT_SYMBOL(clocksource_change_rating);
>   */
>  static int clocksource_unbind(struct clocksource *cs)
>  {
> -	/*
> -	 * I really can't convince myself to support this on hardware
> -	 * designed by lobotomized monkeys.
> -	 */
> -	if (clocksource_is_watchdog(cs))
> -		return -EBUSY;
> +	if (clocksource_is_watchdog(cs)) {
> +		/* Select and try to install a replacement watchdog. */
> +		clocksource_select_watchdog(true);
> +		if (clocksource_is_watchdog(cs))
> +			return -EBUSY;
> +	}
>  
>  	if (cs == curr_clocksource) {
>  		/* Select and try to install a replacement clock source */
> -- 
> 1.9.1
> 
> 
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