linux-kernel - Re: [RFC PATCH v2 2/6] timekeeping: Fix cross-timestamp interpolation corner case decision

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Date:   Thu, 24 Aug 2023 21:02:45 -0700
From:   John Stultz <jstultz@...gle.com>
To:     Peter Hilber <peter.hilber@...nsynergy.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        "Christopher S. Hall" <christopher.s.hall@...el.com>
Subject: Re: [RFC PATCH v2 2/6] timekeeping: Fix cross-timestamp interpolation
 corner case decision

On Thu, Aug 17, 2023 at 6:20 PM Peter Hilber
<peter.hilber@...nsynergy.com> wrote:
>
> 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@...nsynergy.com>

Thanks for respinning these.   It's still a little tough to get my
head around why this is needed, but the extra explanation helps a lot!

Acked-by: John Stultz <jstultz@...gle.com>