linux-kernel - Re: infinite loop in read_hpet from ktime_get_boot_fast

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Message-ID: <CAHmME9oWhWi=Gp2RpM0AOO+_1_24znUxDkz6CyJTc2qRgRRivw@mail.gmail.com>
Date:   Wed, 12 Jun 2019 14:58:21 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, clemens@...isch.de,
        Sultan Alsawaf <sultan@...neltoast.com>,
        Waiman Long <longman@...hat.com>, X86 ML <x86@...nel.org>
Subject: Re: infinite loop in read_hpet from ktime_get_boot_fast_ns

Hi Peter,

Thanks for the explanation.

On Wed, Jun 12, 2019 at 2:29 PM Peter Zijlstra <peterz@...radead.org> wrote:
> Either local_clock() or cpu_clock(cpu). The sleep hooks are not
> something the consumer has to worry about.

Alright. Just so long as it *is* tracking sleep, then that's fine. If
it isn't some important aspects of the protocol will be violated.

> If an architecture doesn't provide a sched_clock(), you're on a
> seriously handicapped arch. It wraps in ~500 days, and aside from
> changing jiffies_lock to a latch, I don't think we can do much about it.

Are you sure? The base definition I'm looking at uses jiffies:

unsigned long long __weak sched_clock(void)
{
        return (unsigned long long)(jiffies - INITIAL_JIFFIES)
                                        * (NSEC_PER_SEC / HZ);
}

On a CONFIG_HZ_1000 machine, jiffies wraps in ~49.7 days:
>>> ((1<<32)-1)/1000/(60*60*24)
49.710269618055555

Why not just use get_jiffies_64()? The lock is too costly on 32bit?

> (the scheduler too expects sched_clock() to not wrap short of the u64
> and so having those machines online for 500 days will get you 'funny'
> results)

Ahh. So if, on the other hand, the whole machine explodes at the wrap
mark, I guess my silly protocol is the least of concerns, and so this
shouldn't matter?

Jason