linux-kernel - Re: [PATCH 1/3] [RFC] hrtimer: Fix clock_was_set so it is safe to call from irq context

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Date:	Thu, 05 Jul 2012 10:29:07 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	John Stultz <johnstul@...ibm.com>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>,
	stable@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	linux@...nhuawei.org
Subject: Re: [PATCH 1/3] [RFC] hrtimer: Fix clock_was_set so it is safe to
 call from irq context



On 07/04/2012 02:21 AM, John Stultz wrote:
> NOTE:This is a prerequisite patch that's required to
> address the widely observed leap-second related futex/hrtimer
> issues.
> 
> Currently clock_was_set() is unsafe to be called from irq
> context, as it calls on_each_cpu(). This causes problems when
> we need to adjust the time from update_wall_time().
> 
> To fix this, if clock_was_set is called when irqs are
> disabled, we schedule a timer to fire for immedately after
> we're out of interrupt context to then notify the hrtimer
> subsystem.
> 
> CC: Prarit Bhargava <prarit@...hat.com>
> CC: stable@...r.kernel.org
> CC: Thomas Gleixner <tglx@...utronix.de>
> CC: linux@...nhuawei.org
> Reported-by: Jan Engelhardt <jengelh@...i.de>
> Signed-off-by: John Stultz <johnstul@...ibm.com>
> ---
>  kernel/hrtimer.c |   17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
> index ae34bf5..d730678 100644
> --- a/kernel/hrtimer.c
> +++ b/kernel/hrtimer.c
> @@ -746,7 +746,7 @@ static inline void retrigger_next_event(void *arg) { }
>   * resolution timer interrupts. On UP we just disable interrupts and
>   * call the high resolution interrupt code.
>   */
> -void clock_was_set(void)
> +static void do_clock_was_set(unsigned long data)
>  {
>  #ifdef CONFIG_HIGH_RES_TIMERS
>  	/* Retrigger the CPU local events everywhere */
> @@ -755,6 +755,21 @@ void clock_was_set(void)
>  	timerfd_clock_was_set();
>  }
>  
> +static DEFINE_TIMER(clock_was_set_timer, do_clock_was_set , 0, 0);
> +
> +void clock_was_set(void)
> +{
> +	/*
> +	 * We can't call on_each_cpu() from irq context,
> +	 * so if irqs are disabled , schedule the clock_was_set
> +	 * via a timer_list timer for right after.
> +	 */
> +	if (irqs_disabled())

I was wondering about this in earlier versions of the patchset.  I thought it
was supposed to be irqs_disabled.  Thanks for changing this.

Acked-by: Prarit Bhargava <prarit@...hat.com>

P.
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