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 3.4-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-3.4 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
@@ -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);
}
Patches currently in stable-queue which might be from js1304@xxxxxxxxx are
queue-3.4/driver-core-treat-unregistered-bus_types-as-having-no-devices.patch
queue-3.4/arm-7643-1-sched-correct-update_sched_clock.patch
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