Commit 87a41130 authored by Peter Hilber's avatar Peter Hilber Committed by Thomas Gleixner
Browse files

timekeeping: Fix cross-timestamp interpolation corner case decision 



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 changing cycle_between() into timestamp_in_interval(), which
now checks against the closed interval, rather than the open interval.

This changes the get_device_system_crosststamp() behavior for three corner
cases:

1. Bypass interpolation in the case cycles == tk->tkr_mono.cycle_last,
   fixing the problem described above.

2. At the first timestamp_in_interval() call site, cycles == now no longer
   causes failure.

3. At the second timestamp_in_interval() call site, history_begin->cycles
   == system_counterval.cycles no longer causes failure.
   adjust_historical_crosststamp() also works for this corner case,
   where partial_history_cycles == total_history_cycles.

These behavioral changes should not cause any problems.

Fixes: 2c756feb ("time: Add history to cross timestamp interface supporting slower devices")
Signed-off-by: default avatarPeter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20231218073849.35294-3-peter.hilber@opensynergy.com
parent 84dccadd

kernel/time/timekeeping.c

+10 −8
Original line number Diff line number Diff line
@@ -1180,13 +1180,15 @@ static int adjust_historical_crosststamp(struct system_time_snapshot *history, 
}



/*

 * cycle_between - true if test occurs chronologically between before and after

 * timestamp_in_interval - true if ts is chronologically in [start, end]

 *

 * True if ts occurs chronologically at or after start, and before or at end.

 */

static bool cycle_between(u64 before, u64 test, u64 after)

static bool timestamp_in_interval(u64 start, u64 end, u64 ts)

{

	if (test > before && test < after)

	if (ts >= start && ts <= end)

		return true;

	if (before > after && (test > before || test < after))

	if (start > end && (ts >= start || ts <= end))

		return true;

	return false;

}
@@ -1246,7 +1248,7 @@ 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 (!timestamp_in_interval(interval_start, now, cycles)) {

			clock_was_set_seq = tk->clock_was_set_seq;

			cs_was_changed_seq = tk->cs_was_changed_seq;

			cycles = interval_start;
@@ -1277,13 +1279,13 @@ int get_device_system_crosststamp(int (*get_time_fn) 
		bool discontinuity;



		/*

		 * Check that the counter value occurs after the provided

		 * Check that the counter value is not before the provided

		 * history reference and that the history doesn't cross a

		 * clocksource change

		 */

		if (!history_begin ||

		    !cycle_between(history_begin->cycles,

				   system_counterval.cycles, cycles) ||

		    !timestamp_in_interval(history_begin->cycles,

					   cycles, system_counterval.cycles) ||

		    history_begin->cs_was_changed_seq != cs_was_changed_seq)

			return -EINVAL;

		partial_history_cycles = cycles - system_counterval.cycles;