linux-kernel - Re: [PATCH] Prevent clockevent event_handler ending up handler

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Date:	Wed, 3 Sep 2008 01:31:48 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
cc:	linux-kernel <linux-kernel@...r.kernel.org>, shaohua.li@...el.com
Subject: Re: [PATCH] Prevent clockevent event_handler ending up
 handler_noop

On Tue, 2 Sep 2008, Venki Pallipadi wrote:
> There is a ordering related problem with clockevents code, due to which
> clockevents_register_device() called after tickless/highres switch
> will not work. The new clockevent ends up with clockevents_handle_noop as
> event handler, resulting in no timer activity.
> 
> The problematic path seems to be
> 
> * old device already has hrtimer_interrupt as the event_handler
> * new clockevent device registers with a higher rating
> * tick_check_new_device() is called
>   * clockevents_exchange_device() gets called
>     * old->event_handler is set to clockevents_handle_noop
>   * tick_setup_device() is called for the new device
>     * which sets new->event_handler using the old->event_handler which is noop.
> 
> Change the ordering so that new device inherits the proper handler.
> 
> This does not have any issue in normal case as most likely all the clockevent
> devices are setup before the highres switch. But, can potentially be affecting
> some corner case where HPET force detect happens after the highres switch.
> This was a problem with HPET in MSI mode code that we have been experimenting
> with.
> 
> Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
> Signed-off-by: Shaohua Li <shaohua.li@...el.com>

Acked-by: Thomas Gleixner <tglx@...utronix.de>

Should go into stable as well.

/me looks for a brown paperbag

Thanks, 

	tglx

> ---
>  include/linux/clockchips.h |    2 ++
>  kernel/time/clockevents.c  |    3 +--
>  kernel/time/tick-common.c  |    1 +
>  3 files changed, 4 insertions(+), 2 deletions(-)
> 
> Index: tip/kernel/time/clockevents.c
> ===================================================================
> --- tip.orig/kernel/time/clockevents.c	2008-09-02 13:26:35.000000000 -0700
> +++ tip/kernel/time/clockevents.c	2008-09-02 15:24:13.000000000 -0700
> @@ -177,7 +177,7 @@ void clockevents_register_device(struct 
>  /*
>   * Noop handler when we shut down an event device
>   */
> -static void clockevents_handle_noop(struct clock_event_device *dev)
> +void clockevents_handle_noop(struct clock_event_device *dev)
>  {
>  }
>  
> @@ -199,7 +199,6 @@ void clockevents_exchange_device(struct 
>  	 * released list and do a notify add later.
>  	 */
>  	if (old) {
> -		old->event_handler = clockevents_handle_noop;
>  		clockevents_set_mode(old, CLOCK_EVT_MODE_UNUSED);
>  		list_del(&old->list);
>  		list_add(&old->list, &clockevents_released);
> Index: tip/kernel/time/tick-common.c
> ===================================================================
> --- tip.orig/kernel/time/tick-common.c	2008-09-02 13:26:35.000000000 -0700
> +++ tip/kernel/time/tick-common.c	2008-09-02 15:24:13.000000000 -0700
> @@ -161,6 +161,7 @@ static void tick_setup_device(struct tic
>  	} else {
>  		handler = td->evtdev->event_handler;
>  		next_event = td->evtdev->next_event;
> +		td->evtdev->event_handler = clockevents_handle_noop;
>  	}
>  
>  	td->evtdev = newdev;
> Index: tip/include/linux/clockchips.h
> ===================================================================
> --- tip.orig/include/linux/clockchips.h	2008-09-02 13:26:34.000000000 -0700
> +++ tip/include/linux/clockchips.h	2008-09-02 15:24:13.000000000 -0700
> @@ -127,6 +127,8 @@ extern int clockevents_register_notifier
>  extern int clockevents_program_event(struct clock_event_device *dev,
>  				     ktime_t expires, ktime_t now);
>  
> +extern void clockevents_handle_noop(struct clock_event_device *dev);
> +
>  #ifdef CONFIG_GENERIC_CLOCKEVENTS
>  extern void clockevents_notify(unsigned long reason, void *arg);
>  #else
> --
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