Re: infinite loop in read_hpet from ktime_get_boot_fast_ns

From: Arnd Bergmann
Date: Wed Jun 12 2019 - 15:51:23 EST

On Wed, Jun 12, 2019 at 7:55 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.