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

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Message-Id: <20230630171052.985577-3-peter.hilber@opensynergy.com>
Date:   Fri, 30 Jun 2023 19:10:45 +0200
From:   Peter Hilber <peter.hilber@...nsynergy.com>
To:     linux-kernel@...r.kernel.org
Cc:     Peter Hilber <peter.hilber@...nsynergy.com>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        "Christopher S. Hall" <christopher.s.hall@...el.com>
Subject: [RFC PATCH 2/7] timekeeping: Fix cross-timestamp interpolation corner case decision

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@...nsynergy.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