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Commit 0209b937 authored by Thomas Graziadei's avatar Thomas Graziadei Committed by John Stultz
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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: default avatarThomas Graziadei <thomas.graziadei@omicronenergy.com>
Signed-off-by: default avatarJohn Stultz <john.stultz@linaro.org>
parent 0fb71d34

kernel/time/timekeeping.c

+6 −4
Original line number Original line Diff line number Diff line
@@ -480,11 +480,13 @@ static inline void old_vsyscall_fixup(struct timekeeper *tk) 
	* users are removed, this can be killed.

	* users are removed, this can be killed.

	*/

	*/

	remainder = tk->tkr_mono.xtime_nsec & ((1ULL << tk->tkr_mono.shift) - 1);

	remainder = tk->tkr_mono.xtime_nsec & ((1ULL << tk->tkr_mono.shift) - 1);

	if (remainder != 0) {

		tk->tkr_mono.xtime_nsec -= remainder;

		tk->tkr_mono.xtime_nsec -= remainder;

		tk->tkr_mono.xtime_nsec += 1ULL << tk->tkr_mono.shift;

		tk->tkr_mono.xtime_nsec += 1ULL << tk->tkr_mono.shift;

		tk->ntp_error += remainder << tk->ntp_error_shift;

		tk->ntp_error += remainder << tk->ntp_error_shift;

		tk->ntp_error -= (1ULL << tk->tkr_mono.shift) << tk->ntp_error_shift;

		tk->ntp_error -= (1ULL << tk->tkr_mono.shift) << tk->ntp_error_shift;

	}

	}

}

#else

#else

#define old_vsyscall_fixup(tk)

#define old_vsyscall_fixup(tk)

#endif

#endif