Commit-ID: 0209b937569a133dedfe930cdfff3a0d1d68c9e9
Gitweb: http://git.kernel.org/tip/0209b937569a133dedfe930cdfff3a0d1d68c9e9
Author: Thomas Graziadei <thomas.graziadei@xxxxxxxxxxxxxxxxx>
AuthorDate: Tue, 31 May 2016 15:06:06 +0200
Committer: John Stultz <john.stultz@xxxxxxxxxx>
CommitDate: Mon, 20 Jun 2016 12:46:45 -0700
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@xxxxxxxxxx>
Cc: Richard Cochran <richardcochran@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Thomas Graziadei <thomas.graziadei@xxxxxxxxxxxxxxxxx>
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
---
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)
--
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