[RFC PATCH 2/7] timekeeping: Fix cross-timestamp interpolation corner case decision

Peter Hilber
2023-06-30T17:33:25Z

cycle_between() decides whether get_device_system_crosststamp() will
interpolate for older counter readings. So far, cycle_between() checks if
parameter test is in the open interval (before, after), when disregarding
the special case before > after.

The only cycle_between() user, get_device_system_crosststamp(), has the
following problem with this: If interval_start == cycles,
cycle_between(interval_start, cycles, now) returns false. If a
history_begin was supplied to get_device_system_crosststamp(), it will
later call cycle_between() again, with effective argument values
cycle_between(history_begin->cycles, cycles, cycles). Due to the test
against the open interval, cycle_between() returns false again, and
get_device_system_crosststamp() returns -EINVAL, when it could have
succeeded.

Fix this by testing against the closed interval in cycle_between(). This
disables interpolation if interval_start == cycles. For the special case
before > after, similar arguments hold. Fix this in a similar way.

At the second cycle_between() call site, add an extra condition in order to
effectively check a half-open interval, which keeps the condition
documented above the call site satisfied.

Fixes: 2c756feb18d9 ("time: Add history to cross timestamp interface supporting slower devices")
Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
---
kernel/time/timekeeping.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 8f35455b6250..7e86d5cd784d 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1180,13 +1180,13 @@ static int adjust_historical_crosststamp(struct system_time_snapshot *history,
}

/*
- * cycle_between - true if test occurs chronologically between before and after
+ * cycle_between - true if test occurs chronologically in [before, after]
*/
static bool cycle_between(u64 before, u64 test, u64 after)
{
- if (test > before && test < after)
+ if (test >= before && test <= after)
return true;
- if (before > after && (test > before || test < after))
+ if (before > after && (test >= before || test <= after))
return true;
return false;
}
@@ -1282,6 +1282,7 @@ int get_device_system_crosststamp(int (*get_time_fn)
* clocksource change
*/
if (!history_begin ||
+ history_begin->cycles == system_counterval.cycles ||
!cycle_between(history_begin->cycles,
system_counterval.cycles, cycles) ||
history_begin->cs_was_changed_seq != cs_was_changed_seq)
--
2.39.2