[PATCH] sched: fix a bug in sched domain degenerate

[Li Zefan <lizf@cn.fujitsu.com>]

(1) on i386 with SCHED_SMT and SCHED_MC enabled
	# mount -t cgroup -o cpuset xxx /mnt
	# echo 0 > /mnt/cpuset.sched_load_balance
	# mkdir /mnt/0
	# echo 0 > /mnt/0/cpuset.cpus
	# dmesg
	CPU0 attaching sched-domain:
	 domain 0: span 0 level CPU
	  groups: 0

(2) on i386 with SCHED_MC enabled but SCHED_SMT disabled
	# same with (1)
	# dmesg
	CPU0 attaching NULL sched-domain.

The bug is some sched domains may be skipped unintentionally when
doing sched domain degenerating.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
---
 kernel/sched.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index dee79ad..b13f45a 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -6875,15 +6875,17 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
 	struct sched_domain *tmp;
 
 	/* Remove the sched domains which do not contribute to scheduling. */
-	for (tmp = sd; tmp; tmp = tmp->parent) {
+	for (tmp = sd; tmp; ) {
 		struct sched_domain *parent = tmp->parent;
 		if (!parent)
 			break;
+
 		if (sd_parent_degenerate(tmp, parent)) {
 			tmp->parent = parent->parent;
 			if (parent->parent)
 				parent->parent->child = tmp;
-		}
+		} else
+			tmp = tmp->parent;
 	}
 
 	if (sd && sd_degenerate(sd)) {
-- 
1.5.4.rc3