[ 60/86] ARM: 7643/1: sched: correct update_sched_clock()

Greg Kroah-Hartman
2013-02-27T00:09:20Z

From: Joonsoo Kim <js1304@gmail.com>

commit 7c4e9ced424be4d36df6a3e3825763e97ee97607 upstream.

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@lge.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/arm/kernel/sched_clock.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/arch/arm/kernel/sched_clock.c
+++ b/arch/arm/kernel/sched_clock.c
@@ -84,11 +84,11 @@ static void notrace update_sched_clock(v
* 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);
}