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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>
This commit is contained in:
Thomas Graziadei 2016-05-31 15:06:06 +02:00 committed by John Stultz
parent 0fb71d340d
commit 0209b93756
1 changed files with 6 additions and 4 deletions
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10
kernel/time/timekeeping.c
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@ -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)