linux-kernel - Re: [PATCH 1/2] kernel, timekeeping, add trylock option to ktime_get_with

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Date:	Wed, 06 Jan 2016 14:06:02 -0500
From:	Prarit Bhargava <prarit@...hat.com>
To:	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	lkml <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Xunlei Pang <pang.xunlei@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Baolin Wang <baolin.wang@...aro.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 1/2] kernel, timekeeping, add trylock option to ktime_get_with_offset()



On 01/06/2016 02:04 PM, John Stultz wrote:
> On Wed, Jan 6, 2016 at 9:34 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> On Wed, 6 Jan 2016, John Stultz wrote:
>>> On Wed, Jan 6, 2016 at 5:00 AM, Prarit Bhargava <prarit@...hat.com> wrote:
>>>> -ktime_t ktime_get_with_offset(enum tk_offsets offs)
>>>> +ktime_t ktime_get_with_offset(enum tk_offsets offs, int trylock)
>>>>  {
>>>>         struct timekeeper *tk = &tk_core.timekeeper;
>>>>         unsigned int seq;
>>>>         ktime_t base, *offset = offsets[offs];
>>>>         s64 nsecs;
>>>> +       unsigned long flags = 0;
>>>> +
>>>> +       if (unlikely(!timekeeping_initialized))
>>>> +               return ktime_set(0, 0);
>>>>
>>>>         WARN_ON(timekeeping_suspended);
>>>>
>>>> +       if (trylock && !raw_spin_trylock_irqsave(&timekeeper_lock, flags))
>>>> +               return ktime_set(KTIME_MAX, 0);
>>>
>>> Wait.. this doesn't make sense. The timekeeper lock is only for reading.
>>>
>>> What I was suggesting to you off line is to have something that avoids
>>> spinning on the seqcounter should if a bug occurs and we IPI all the
>>> cpus, that we don't deadlock or block any printk messages.
>>
>> We could also extend the fast timekeeper with boot/real/tai extensions and use
>> that for printk. You can use ktime_get_mono_fast_ns() today.
> 
> Ack. There'd be a chance for odd values around when the time is set,
> but for debug printks I think its not critical.

I'll convert to this in v2.

Thanks for the input everyone!

P.

> 
> thanks
> -john
> 
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