From db0f5de3c8c38c4a59b829c0117440bb851fbe0d Mon Sep 17 00:00:00 2001 From: Joonsoo Kim 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. cyc = 9 ns = 900 cyc_copy = 9 == CASE 1 == 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 == 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 Signed-off-by: Russell King Signed-off-by: Greg Kroah-Hartman --- 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 fc6692e..bd6f56b 100644 --- a/arch/arm/kernel/sched_clock.c +++ b/arch/arm/kernel/sched_clock.c @@ -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); } -- 2.7.4