[RFC][PATCH] cpusets: set task's cpu_allowed to cpu_possible_map when attaching it into top cpuset

[Miao Xie <miaox@cn.fujitsu.com>]

I found a bug on my dual-cpu box. I created a sub cpuset in top cpuset and
assign 1 to its cpus. And then we attach some tasks into this sub cpuset.
After this, we offline CPU1. Now, the tasks in this new cpuset are moved into
top cpuset automatically because there is no cpu in sub cpuset. Then we
online CPU1, we find all the tasks which doesn't belong to top cpuset originally
just run on CPU0.

We fix this bug by setting task's cpu_allowed to cpu_possible_map when attaching
it into top cpuset. This method needn't modify the current behavior of cpusets on
CPU hotplug, and all of tasks in top cpuset use cpu_possible_map to initialize
their cpu_allowed.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>

---
 kernel/cpuset.c |   10 +++++++---
 1 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 96c0ba1..e087b72 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1332,10 +1332,14 @@ static void cpuset_attach(struct cgroup_subsys *ss,
 	struct cpuset *oldcs = cgroup_cs(oldcont);
 	int err;
 
-	mutex_lock(&callback_mutex);
-	guarantee_online_cpus(cs, &cpus);
+	if (cs == &top_cpuset) {
+		cpus = cpu_possible_map;
+	} else {
+		mutex_lock(&callback_mutex);
+		guarantee_online_cpus(cs, &cpus);
+		mutex_unlock(&callback_mutex);
+	}
 	err = set_cpus_allowed_ptr(tsk, &cpus);
-	mutex_unlock(&callback_mutex);
 	if (err)
 		return;
 
-- 1.5.4.rc3