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

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Date:	Wed, 6 Jan 2016 11:04:16 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Prarit Bhargava <prarit@...hat.com>,
	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 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.

thanks
-john
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