powerpc64 head -r344018 stuck sleeping problems: th->th_scale * tc_delta(th) overflows unsigned 64 bits sometimes [patched failed]

[Konstantin Belousov]

On Sat, Apr 06, 2019 at 01:01:19AM +1100, Bruce Evans wrote:
> On Fri, 5 Apr 2019, Konstantin Belousov wrote:
> 
> > On Fri, Apr 05, 2019 at 11:52:27PM +1100, Bruce Evans wrote:
> >> On Fri, 5 Apr 2019, Konstantin Belousov wrote:
> >>
> >>> On Thu, Apr 04, 2019 at 02:47:34AM +1100, Bruce Evans wrote:
> >>>> I noticed (or better realized) a general problem with multiple
> >>>> timehands.  ntpd can slew the clock at up to 500 ppm, and at least an
> >>>> old version of it uses a rate of 50 ppm to fix up fairly small drifts
> >>>> in the milliseconds range.  500 ppm is enormous in CPU cycles -- it is
> >>>> 500 thousand nsec or 2 million cycles at 4GHz.  Winding up the timecounter
> >>>> every 1 msec reduces this to only 2000 cycles.
> >>>> ...
> >>>> The main point of having multiple timehands (after introducing the per-
> >>>> timehands generation count) is to avoid blocking thread N during the
> >>>> update, but this doesn't actually work, even for only 2 timehands and
> >>>> a global generation count.
> >>>
> >>> You are describing the generic race between reader and writer. The same
> >>> race would exist even with one timehand (and/or one global generation
> >>> counter), where ntp adjustment might come earlier or later of some
> >>> consumer accessing the timehands. If timehand instance was read before
> >>> tc_windup() run but code consumed the result after the windup, it might
> >>> appear as if time went backward, and this cannot be fixed without either
> >>> re-reading the time after time-depended calculations were done and
> >>> restarting, or some globabl lock ensuring serialization.
> >>
> >> With 1 timehand, its generation count would be global.  I think its ordering
> >> is strong enough to ensure serialization.
> > Yes, single timehands result in global generation.  But it would not solve
> > the same race appearing in slightly different manner, as I described above.
> > If reader finished while generation number in th was not yet reset, but
> > caller uses the result after tc_windup(), the effect is same as if we
> > have two th's and reader used the outdated one.
> 
> You described it too concisely for me to understand :-).
> 
> I now see that a single generation count doesn't give serialization.  I
> thought that setting the generation to 0 at the start of tc_windup()
> serialized the reader and writer.  But all it does is prevent use of the
> results of the windup while only some of them are visible.  If the
> setting the generation count to 0 doesn't become before tc_windup() reads
> the hardware timecounter, then this read may be before other reads using
> the old timehands, but it needs to be after.
If we have either single th or global gen counter, current code would
become serialized, but this is not what I am about.  Lets assume, for
the sake of the discussion only, that all readers take the same spinlock
as tc_windup (i.e. tc_setclock_mtx).

It is still possible that reader unlocked the mutex, tc_windup() kicked
in, locked the mutex and moved timehands (as you noted, this might
even happen on the same CPU), and only then the reader continues. For
consumer of bintime() or any other function' result, it looks exactly
the same as if we did not serialized with writer but used outdated
timehands.

Let me formulate this diffeently: as far as consumer of the bintime()
result does not serialize itself with tc_windup(), serializing bintime()
itself against tc_windup() does not close the race, but it is not
obvious that the race matters. Either we should just accept the race as
we currently do, or readers must take the spinlock where the exact value
of the current time is important, or readers must re-read the time after
doing something important, and redo if the new measuremedtime is outside
the acceptable range.

> 
> A not so good fix for this is to wait a bit after setting the generation
> count to 0, so that the change becomes visible on all CPUs.
> 
> Bruce