Re: [PATCH 3/6] timekeeping: Make it safe to use the fast timekeeper while suspended

[Rafael J. Wysocki]

On Friday, February 13, 2015 08:53:38 AM John Stultz wrote:
> On Wed, Feb 11, 2015 at 12:03 PM, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> >
> > Theoretically, ktime_get_mono_fast_ns() may be executed after
> > timekeeping has been suspended (or before it is resumed) which
> > in turn may lead to undefined behavior, for example, when the
> > clocksource read from timekeeping_get_ns() called by it is
> > not accessible at that time.
> 
> And the callers of the ktime_get_mono_fast_ns() have to get back a
> value?

Yes, they do.

> Or can we return an error on timekeeping_suspended like we do
> w/ __getnstimeofday64()?

No, we can't.

> Also, what exactly is the case when the clocksource being read isn't
> accessible? I see this is conditionalized on
> CLOCK_SOURCE_SUSPEND_NONSTOP, so is the concern on resume we read the
> clocksource and its been reset causing a crazy time value?

The clocksource's ->suspend method may have been called (during suspend)
and depending on what that did we may even crash things theoretically.

During resume, before the clocksource's ->resume callback, it may just
be undefined behavior (random data etc).

For system suspend as we have today the window is quite narrow, but after
patch [4/6] from this series suspend-to-idle may suspend timekeeping and
just sit there in idle for extended time (hours even) which broadens the
potential exposure quite a bit.

Of course, it does that with interrupts disabled, but ktime_get_mono_fast_ns()
is for NMI, so theoretically, if an NMI happens while we're in suspend-to-idle
with timekeeping suspended and the clocksource is not CLOCK_SOURCE_SUSPEND_NONSTOP
and the NMI calls ktime_get_mono_fast_ns(), strange and undesirable things may
happen.

> > Prevent that from happening by setting up a dummy readout base for
> > the fast timekeeper during timekeeping_suspend() such that it will
> > always return the same number of cycles.
> >
> > After the last timekeeping_update() in timekeeping_suspend() the
> > clocksource is read and the result is stored as cycles_at_suspend.
> > The readout base from the current timekeeper is copied onto the
> > dummy and the ->read pointer of the dummy is set to a routine
> > unconditionally returning cycles_at_suspend.  Next, the dummy is
> > passed to update_fast_timekeeper().
> >
> > Then, ktime_get_mono_fast_ns() will work until the subsequent
> > timekeeping_resume() and the proper readout base for the fast
> > timekeeper will be restored by the timekeeping_update() called
> > right after clearing timekeeping_suspended.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > ---
> >  kernel/time/timekeeping.c |   22 ++++++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> >
> > Index: linux-pm/kernel/time/timekeeping.c
> > ===================================================================
> > --- linux-pm.orig/kernel/time/timekeeping.c
> > +++ linux-pm/kernel/time/timekeeping.c
> > @@ -1249,9 +1249,23 @@ static void timekeeping_resume(void)
> >         hrtimers_resume();
> >  }
> >
> > +/*
> > + * Dummy readout base and suspend-time cycles value for the fast timekeeper to
> > + * work in a consistent way after timekeeping has been suspended if the core
> > + * timekeeper clocksource is not suspend-nonstop.
> > + */
> > +static struct tk_read_base tkr_dummy;
> > +static cycle_t cycles_at_suspend;
> > +
> > +static cycle_t dummy_clock_read(struct clocksource *cs)
> > +{
> > +       return cycles_at_suspend;
> > +}
> > +
> >  static int timekeeping_suspend(void)
> >  {
> >         struct timekeeper *tk = &tk_core.timekeeper;
> > +       struct clocksource *clock = tk->tkr.clock;
> >         unsigned long flags;
> >         struct timespec64               delta, delta_delta;
> >         static struct timespec64        old_delta;
> > @@ -1294,6 +1308,14 @@ static int timekeeping_suspend(void)
> >         }
> >
> >         timekeeping_update(tk, TK_MIRROR);
> > +
> > +       if (!(clock->flags & CLOCK_SOURCE_SUSPEND_NONSTOP)) {
> > +               memcpy(&tkr_dummy, &tk->tkr, sizeof(tkr_dummy));
> > +               cycles_at_suspend = tk->tkr.read(clock);
> > +               tkr_dummy.read = dummy_clock_read;
> > +               update_fast_timekeeper(&tkr_dummy);
> > +       }
> 
> Its a little ugly... though I'm not sure I have a better idea right off.
> 
> thanks
> -john
> --
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-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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