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    SubjectRe: [PATCH 3/6] timekeeping: Make it safe to use the fast timekeeper while suspended
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    On Wed, Feb 11, 2015 at 12:03 PM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
    > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    >
    > 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? Or can we return an error on timekeeping_suspended like we do
    w/ __getnstimeofday64()?

    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?

    > 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@intel.com>
    > ---
    > 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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