[patch 20/28] ntp: fix calculation of the next jiffie to trigger RTC sync

[Greg KH <gregkh@suse.de>]

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2.6.25-stable review patch.  If anyone has any objections, please let us
know.

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From: Maciej W. Rozycki <macro@linux-mips.org>

commit 4ff4b9e19a80b73959ebeb28d1df40176686f0a8 upstream

We have a bug in the calculation of the next jiffie to trigger the RTC
synchronisation.  The aim here is to run sync_cmos_clock() as close as
possible to the middle of a second.  Which means we want this function to
be called less than or equal to half a jiffie away from when now.tv_nsec
equals 5e8 (500000000).

If this is not the case for a given call to the function, for this purpose
instead of updating the RTC we calculate the offset in nanoseconds to the
next point in time where now.tv_nsec will be equal 5e8.  The calculated
offset is then converted to jiffies as these are the unit used by the
timer.

Hovewer timespec_to_jiffies() used here uses a ceil()-type rounding mode,
where the resulting value is rounded up.  As a result the range of
now.tv_nsec when the timer will trigger is from 5e8 to 5e8 + TICK_NSEC
rather than the desired 5e8 - TICK_NSEC / 2 to 5e8 + TICK_NSEC / 2.

As a result if for example sync_cmos_clock() happens to be called at the
time when now.tv_nsec is between 5e8 + TICK_NSEC / 2 and 5e8 to 5e8 +
TICK_NSEC, it will simply be rescheduled HZ jiffies later, falling in the
same range of now.tv_nsec again.  Similarly for cases offsetted by an
integer multiple of TICK_NSEC.

This change addresses the problem by subtracting TICK_NSEC / 2 from the
nanosecond offset to the next point in time where now.tv_nsec will be
equal 5e8, effectively shifting the following rounding in
timespec_to_jiffies() so that it produces a rounded-to-nearest result.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
 kernel/time/ntp.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -205,7 +205,7 @@ static void sync_cmos_clock(unsigned lon
 	if (abs(now.tv_nsec - (NSEC_PER_SEC / 2)) <= tick_nsec / 2)
 		fail = update_persistent_clock(now);
 
-	next.tv_nsec = (NSEC_PER_SEC / 2) - now.tv_nsec;
+	next.tv_nsec = (NSEC_PER_SEC / 2) - now.tv_nsec - (TICK_NSEC / 2);
 	if (next.tv_nsec <= 0)
 		next.tv_nsec += NSEC_PER_SEC;
 

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