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

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Message-ID: <87lh0urdyu.fsf@gmail.com>
Date:	Thu, 21 Jul 2016 21:24:57 +0200
From:	Nicolai Stange <nicstange@...il.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Nicolai Stange <nicstange@...il.com>,
	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

John Stultz <john.stultz@...aro.org> writes:

> 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?

Yes, but the first call is made under a different lock than the second
one.

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


Thanks,

Nicolai