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    Subject[RFC PATCH v2 2/6] timekeeping: Fix cross-timestamp interpolation corner case decision
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    The cycle_between() helper checks if parameter test is in the open interval
    (before, after). Colloquially speaking, this also applies to the counter
    wrap-around special case before > after. get_device_system_crosststamp()
    currently uses cycle_between() at the first call site to decide whether to
    interpolate for older counter readings.

    get_device_system_crosststamp() has the following problem with
    cycle_between() testing against an open interval: Assume that, by chance,
    cycles == tk->tkr_mono.cycle_last (in the following, "cycle_last" for
    brevity). Then, cycle_between() at the first call site, with effective
    argument values cycle_between(cycle_last, cycles, now), returns false,
    enabling interpolation. During interpolation,
    get_device_system_crosststamp() will then call cycle_between() at the
    second call site (if a history_begin was supplied). The effective argument
    values are cycle_between(history_begin->cycles, cycles, cycles), since
    system_counterval.cycles == interval_start == cycles, per the assumption.
    Due to the test against the open interval, cycle_between() returns false
    again. This causes get_device_system_crosststamp() to return -EINVAL.

    This failure should be avoided, since get_device_system_crosststamp() works
    both when cycles follows cycle_last (no interpolation), and when cycles
    precedes cycle_last (interpolation). For the case cycles == cycle_last,
    interpolation is actually unneeded.

    Fix this by disabling interpolation if cycles == cycle_last. Thereby, avoid
    the above described corner case interpolation failure.

    Fixes: 2c756feb18d9 ("time: Add history to cross timestamp interface supporting slower devices")
    Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
    ---

    Notes:
    v2:

    - try to clarify problem description (John Stultz)
    - simplify fix

    kernel/time/timekeeping.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
    index cd5c83473bab..70ecd44fdd9e 100644
    --- a/kernel/time/timekeeping.c
    +++ b/kernel/time/timekeeping.c
    @@ -1247,7 +1247,8 @@ int get_device_system_crosststamp(int (*get_time_fn)
    */
    now = tk_clock_read(&tk->tkr_mono);
    interval_start = tk->tkr_mono.cycle_last;
    - if (!cycle_between(interval_start, cycles, now)) {
    + if (!cycle_between(interval_start, cycles, now) &&
    + cycles != interval_start) {
    clock_was_set_seq = tk->clock_was_set_seq;
    cs_was_changed_seq = tk->cs_was_changed_seq;
    cycles = interval_start;
    --
    2.39.2
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