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>

1 files changed, 5 insertions, 2 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index b53785346850..796baf731976 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;
 }
 
 /**