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

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.

Cc: <stable@vger.kernel.org>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
This commit is contained in:
Joonsoo Kim 2013-02-09 05:52:45 +01:00 committed by Russell King
parent 836dc9e3fb
commit 7c4e9ced42
1 changed files with 2 additions and 2 deletions
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4
arch/arm/kernel/sched_clock.c
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@ -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);
}