linux-kernel - [PATCH] timekeeping: Fix 1ns/tick drift with GENERIC_TIME_VSYSCALL

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Message-ID: <1464699966-26521-1-git-send-email-thomas.graziadei@omicronenergy.com>
Date:	Tue, 31 May 2016 15:06:06 +0200
From:	Thomas Graziadei <thomas.graziadei@...cronenergy.com>
To:	<linux-kernel@...r.kernel.org>
CC:	<john.stultz@...aro.org>, <tglx@...utronix.de>,
	<thomas.graziadei@...cronenergy.com>
Subject: [PATCH] timekeeping: Fix 1ns/tick drift with GENERIC_TIME_VSYSCALL_OLD

From: Thomas Graziadei <thomas.graziadei@...cronenergy.com>

The user notices the problem in a raw and real time drift, calling
clock_gettime with CLOCK_REALTIME / CLOCK_MONOTONIC_RAW on a system
with no ntp correction taking place (no ntpd or ptp stuff running).

The problem is, that old_vsyscall_fixup adds an extra 1ns even though
xtime_nsec is already held in full nsecs and the remainder in this
case is 0. Do the rounding up buisness only if needed.

Signed-off-by: Thomas Graziadei <thomas.graziadei@...cronenergy.com>
---
 kernel/time/timekeeping.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 479d25c..a196e08 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -480,10 +480,12 @@ static inline void old_vsyscall_fixup(struct timekeeper *tk)
 	* users are removed, this can be killed.
 	*/
 	remainder = tk->tkr_mono.xtime_nsec & ((1ULL << tk->tkr_mono.shift) - 1);
-	tk->tkr_mono.xtime_nsec -= remainder;
-	tk->tkr_mono.xtime_nsec += 1ULL << tk->tkr_mono.shift;
-	tk->ntp_error += remainder << tk->ntp_error_shift;
-	tk->ntp_error -= (1ULL << tk->tkr_mono.shift) << tk->ntp_error_shift;
+	if (remainder != 0) {
+		tk->tkr_mono.xtime_nsec -= remainder;
+		tk->tkr_mono.xtime_nsec += 1ULL << tk->tkr_mono.shift;
+		tk->ntp_error += remainder << tk->ntp_error_shift;
+		tk->ntp_error -= (1ULL << tk->tkr_mono.shift) << tk->ntp_error_shift;
+	}
 }
 #else
 #define old_vsyscall_fixup(tk)
-- 
1.9.1