From 0209b937569a133dedfe930cdfff3a0d1d68c9e9 Mon Sep 17 00:00:00 2001 From: Thomas Graziadei Date: Tue, 31 May 2016 15:06:06 +0200 Subject: [PATCH] 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 Cc: Richard Cochran Cc: Thomas Gleixner Cc: Ingo Molnar Signed-off-by: Thomas Graziadei Signed-off-by: John Stultz --- 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 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) -- 2.20.1