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

[Peter Hilber]

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@xxxxxxxxxxxxxxx>
---

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