[ 3.5.y.z extended stable ] Patch "ARM: 7643/1: sched: correct update_sched_clock()" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

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

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 50a9827bffa5ddd18f72f5b09526775c1b9e068d Mon Sep 17 00:00:00 2001
From: Joonsoo Kim <js1304 at gmail.com>
Date: Sat, 9 Feb 2013 05:52:45 +0100
Subject: [PATCH] ARM: 7643/1: sched: correct update_sched_clock()

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 at lge.com>
Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 arch/arm/kernel/sched_clock.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm/kernel/sched_clock.c b/arch/arm/kernel/sched_clock.c
index 27d186a..6bbf936 100644
--- a/arch/arm/kernel/sched_clock.c
+++ b/arch/arm/kernel/sched_clock.c
@@ -84,11 +84,11 @@ 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);
 }

--
1.8.1.2