[PATCH RESEND] ARM: sched: correct update_sched_clock()

[Joonsoo Kim]

If we want load epoch_cyc and epoch_ns atomically,
we should update epoch_cyc_copy first of all.
This notify reader that updating is in progress.

If we update epoch_cyc first like as current implementation,
there is subtle error case.
Look at the below example.

<Initial Condition>
cyc = 9
ns = 900
cyc_copy = 9

== CASE 1 ==
<CPU A = reader>           <CPU B = updater>
                           write cyc = 10
read cyc = 10
read ns = 900
                           write ns = 1000
                           write cyc_copy = 10
read cyc_copy = 10

output = (10, 900)

== CASE 2 ==
<CPU A = reader>           <CPU B = updater>
read cyc = 9
                           write cyc = 10
                           write ns = 1000
read ns = 1000
read cyc_copy = 9
                           write cyc_copy = 10
output = (9, 1000)

If atomic read is ensured, output should be (9, 900) or (10, 1000).
But, output in example case are not.

So, change updating sequence in order to correct this problem.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

diff --git a/arch/arm/kernel/sched_clock.c b/arch/arm/kernel/sched_clock.c
index fc6692e..bd6f56b 100644
--- a/arch/arm/kernel/sched_clock.c
+++ b/arch/arm/kernel/sched_clock.c
@@ -93,11 +93,11 @@ static void notrace update_sched_clock(void)
 	 * detectable in cyc_to_fixed_sched_clock().
 	 */
 	raw_local_irq_save(flags);
-	cd.epoch_cyc = cyc;
+	cd.epoch_cyc_copy = cyc;
 	smp_wmb();
 	cd.epoch_ns = ns;
 	smp_wmb();
-	cd.epoch_cyc_copy = cyc;
+	cd.epoch_cyc = cyc;
 	raw_local_irq_restore(flags);
 }
 
-- 
1.7.9.5

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