[tip:sched/balancing] sched: Fix dynamic power-balancing crash

[tip-bot for Ingo Molnar <mingo@elte.hu>]

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;
 }
 
 /**