Patch "ARM: 7643/1: sched: correct update_sched_clock()" has been added to the 3.8-stable tree

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This is a note to let you know that I've just added the patch titled

    ARM: 7643/1: sched: correct update_sched_clock()

to the 3.8-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     arm-7643-1-sched-correct-update_sched_clock.patch
and it can be found in the queue-3.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


>From 7c4e9ced424be4d36df6a3e3825763e97ee97607 Mon Sep 17 00:00:00 2001
From: Joonsoo Kim <js1304@xxxxxxxxx>
Date: Sat, 9 Feb 2013 05:52:45 +0100
Subject: ARM: 7643/1: sched: correct update_sched_clock()

From: Joonsoo Kim <js1304@xxxxxxxxx>

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@xxxxxxx>
Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 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
@@ -93,11 +93,11 @@ 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);
 }
 


Patches currently in stable-queue which might be from js1304@xxxxxxxxx are

queue-3.8/driver-core-treat-unregistered-bus_types-as-having-no-devices.patch
queue-3.8/arm-7643-1-sched-correct-update_sched_clock.patch
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