timekeeping: Fix 1ns/tick drift with GENERIC_TIME_VSYSCALL_OLD

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.

Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Graziadei <thomas.graziadei@omicronenergy.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>

1 files changed, 6 insertions, 4 deletions

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 479d25cd3d4f..a196e08324e7 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)