From: Peter Hilber peter.hilber@opensynergy.com
[ Upstream commit 87a41130881995f82f7adbafbfeddaebfb35f0ef ]
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: 2c756feb18d9 ("time: Add history to cross timestamp interface supporting slower devices") Signed-off-by: Peter Hilber peter.hilber@opensynergy.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Link: https://lore.kernel.org/r/20231218073849.35294-3-peter.hilber@opensynergy.co... Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/time/timekeeping.c | 18 ++++++++++-------- 1 file changed, 10 insertions(+), 8 deletions(-)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 6a8a610b19717..6f615db414769 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -1167,13 +1167,15 @@ 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; } @@ -1233,7 +1235,7 @@ 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; @@ -1264,13 +1266,13 @@ 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;