Commit 5b7dba4f authored Oct 09, 2008 by Dave Kleikamp Committed by Ingo Molnar Oct 10, 2008

sched_clock: prevent scd->clock from moving backwards



When sched_clock_cpu() couples the clocks between two cpus, it may
increment scd->clock beyond the GTOD tick window that __update_sched_clock()
uses to clamp the clock.  A later call to __update_sched_clock() may move
the clock back to scd->tick_gtod + TICK_NSEC, violating the clock's
monotonic property.

This patch ensures that scd->clock will not be set backward.

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent a5d8c348

kernel/sched_clock.c

+3 −3

Original line number	Diff line number	Diff line
		@@ -119,12 +119,12 @@ static u64 __update_sched_clock(struct sched_clock_data *scd, u64 now)
		/*
		* scd->clock = clamp(scd->tick_gtod + delta,
		* max(scd->tick_gtod, scd->clock),
		* scd->tick_gtod + TICK_NSEC);
		* max(scd->clock, scd->tick_gtod + TICK_NSEC));
		*/

		clock = scd->tick_gtod + delta;
		min_clock = wrap_max(scd->tick_gtod, scd->clock);
		max_clock = scd->tick_gtod + TICK_NSEC;
		max_clock = wrap_max(scd->clock, scd->tick_gtod + TICK_NSEC);

		clock = wrap_max(clock, min_clock);
		clock = wrap_min(clock, max_clock);

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