Messages in this thread Patch in this message | | | From | Peter Hilber <> | Subject | [RFC PATCH v2 2/6] timekeeping: Fix cross-timestamp interpolation corner case decision | Date | Fri, 18 Aug 2023 03:20:08 +0200 |
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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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