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Date: Wed, 6 Jan 2016 09:33:27 -0800
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Subject: Re: [PATCH 1/2] kernel, timekeeping, add trylock option to ktime_get_with_offset()
From: John Stultz <john.stultz@linaro.org>
To: Prarit Bhargava <prarit@redhat.com>
Cc: lkml <linux-kernel@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Xunlei Pang <pang.xunlei@linaro.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Baolin Wang <baolin.wang@linaro.org>, Arnd Bergmann <arnd@arndb.de>
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On Wed, Jan 6, 2016 at 9:28 AM, John Stultz <john.stultz@linaro.org> wrote:
> On Wed, Jan 6, 2016 at 5:00 AM, Prarit Bhargava <prarit@redhat.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