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

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Date:   Wed, 12 Jun 2019 21:46:25 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Clemens Ladisch <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

On Wed, Jun 12, 2019 at 7:55 PM Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Jun 12, 2019 at 11:44:35AM +0200, Jason A. Donenfeld wrote:

> > But there's still the
> > issue of the 32-bit wraparound on the base implementation.
>
> 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.
>
> (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)
>
> AFAICT only: alpha, h8300, hexagon, m68knommu, nds32, nios2, openrisc
> are lacking any form of sched_clock(), the rest has it either natively
> or through sched_clock_register().

For completeness (as we already discussed on IRC), on many architectures
this would depend on the clocksource driver: many (older) arm, mips, sh
or m68k implementations don't have sched_clock(), as this depends on
the clocksource driver. All the modern ones tend to have one, but older
ones may only support an interval timer tick that cannot be read.

        Arnd