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Subject[tip:sched/balancing] sched: Fix dynamic power-balancing crash
Commit-ID:  d7ea17a76916e456fcc78e45142c66f7fb875e3d
Gitweb: http://git.kernel.org/tip/d7ea17a76916e456fcc78e45142c66f7fb875e3d
Author: Ingo Molnar <mingo@elte.hu>
AuthorDate: Fri, 4 Sep 2009 11:49:25 +0200
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 4 Sep 2009 11:52:52 +0200

sched: Fix dynamic power-balancing crash

This crash:

[ 1774.088275] divide error: 0000 [#1] SMP
[ 1774.100355] CPU 13
[ 1774.102498] Modules linked in:
[ 1774.105631] Pid: 30881, comm: hackbench Not tainted 2.6.31-rc8-tip-01308-g484d664-dirty #1629 X8DTN
[ 1774.114807] RIP: 0010:[<ffffffff81041c38>] [<ffffffff81041c38>]
sched_balance_self+0x19b/0x2d4

Triggers because update_group_power() modifies the sd tree and does
temporary calculations there - not considering that other CPUs
could observe intermediate values, such as the zero initial value.

Calculate it in a temporary variable instead. (we need no memory
barrier as these are all statistical values anyway)

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <20090904092742.GA11014@elte.hu>
Signed-off-by: Ingo Molnar <mingo@elte.hu>


---
kernel/sched.c | 7 +++++--
1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index b537853..796baf7 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3765,19 +3765,22 @@ static void update_group_power(struct sched_domain *sd, int cpu)
{
struct sched_domain *child = sd->child;
struct sched_group *group, *sdg = sd->groups;
+ unsigned long power;

if (!child) {
update_cpu_power(sd, cpu);
return;
}

- sdg->cpu_power = 0;
+ power = 0;

group = child->groups;
do {
- sdg->cpu_power += group->cpu_power;
+ power += group->cpu_power;
group = group->next;
} while (group != child->groups);
+
+ sdg->cpu_power = power;
}

/**

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