linux-kernel - Re: [RFC v3 2/3] kernel/time/clockevents: make setting of ->mult and ->mult

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Date:	Thu, 21 Jul 2016 11:16:10 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Nicolai Stange <nicstange@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v3 2/3] kernel/time/clockevents: make setting of ->mult and
 ->mult_mono atomic

On Wed, Jul 13, 2016 at 6:00 AM, Nicolai Stange <nicstange@...il.com> wrote:
> In order to avoid races between setting a struct clock_event_device's
> ->mult_mono in clockevents_update_freq() and yet to be implemented updates
> triggered from the timekeeping core, the setting of ->mult and ->mult_mono
> should be made atomic.
>
> Protect the update in clockevents_update_freq() by locking the
> clockevents_lock spinlock. Frequency updates are expected to be done
> seldomly and thus, taking this subsystem lock should not have any impact
> on performance.
>
> Use a raw_spin_lock_irq_save()/raw_spin_unlock_irq_restore() pair for
> locking/unlocking the clockevents_lock spinlock.
> Purge the now redundant local_irq_save()/local_irq_restore() pair from
> clockevents_update_freq(). Since the call to tick_broadcast_update_freq()
> isn't done with interrupts disabled anymore,  its
> raw_spin_lock()/raw_spin_unlock() pair must be converted to
> raw_spin_lock_irq_save()/raw_spin_unlock_irq_restore().
>
> Signed-off-by: Nicolai Stange <nicstange@...il.com>
> ---
>  kernel/time/clockevents.c    | 7 ++++---
>  kernel/time/tick-broadcast.c | 5 +++--
>  2 files changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
> index ba7fea4..ec01375 100644
> --- a/kernel/time/clockevents.c
> +++ b/kernel/time/clockevents.c
> @@ -589,11 +589,12 @@ int clockevents_update_freq(struct clock_event_device *dev, u32 freq)
>         unsigned long flags;
>         int ret;
>
> -       local_irq_save(flags);
>         ret = tick_broadcast_update_freq(dev, freq);
> -       if (ret == -ENODEV)
> +       if (ret == -ENODEV) {
> +               raw_spin_lock_irqsave(&clockevents_lock, flags);
>                 ret = __clockevents_update_freq(dev, freq);
> -       local_irq_restore(flags);
> +               raw_spin_unlock_irqrestore(&clockevents_lock, flags);
> +       }
>         return ret;
>  }
>
> diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
> index f6aae79..9c94c41 100644
> --- a/kernel/time/tick-broadcast.c
> +++ b/kernel/time/tick-broadcast.c
> @@ -125,11 +125,12 @@ int tick_is_broadcast_device(struct clock_event_device *dev)
>  int tick_broadcast_update_freq(struct clock_event_device *dev, u32 freq)
>  {
>         int ret = -ENODEV;
> +       unsigned long flags;
>
>         if (tick_is_broadcast_device(dev)) {
> -               raw_spin_lock(&tick_broadcast_lock);
> +               raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
>                 ret = __clockevents_update_freq(dev, freq);
> -               raw_spin_unlock(&tick_broadcast_lock);
> +               raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
>         }


So not necessarily part of your change, but this makes using
tick_broadcast_update_freq() seem strange.

We call it and if dev is a broadcast_device we call
__clockevents_update_freq(), and if not, it fails and we then just
call __clockevents_update_freq() again?

Why bother calling tick_broadcast_update_freq here, and instead just
call __clockevents_update_freq() directly the first time?

thanks
-john