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

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Date:	Wed, 6 Jan 2016 09:33:27 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	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 9:28 AM, John Stultz <john.stultz@...aro.org> 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.

Only for writing.. sorry.. still drinking my coffee.

> 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.

And more clearly here, if a cpu takes a write on the seqcounter in
update_wall_time() and at that point another cpu hits a bug, and IPIs
the cpus, the system would deadlock. That's really what I want to
avoid.

thanks
-john
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