[PATCH] cgroup: cpuset: fix panic when offline a cpu

[shuwang@redhat.com]

From: Shu Wang <shuwang@redhat.com>

cgroup_migrate assumes mgctx tset.csets is pointing to
tset.src_csets at start, add tasks to tset.src_csets in
func cgroup_migrate_add_task, change test.csets to
tset.dst_csets in cgroup_migrate_execute.

For offline a cpu in cgroup_transfer_tasks, it will first
migrate a task and cause tset.csets pointing to dst_csets.
Get a NULL pointer in cgroup_taskset_first.

reproducer on my 2 cpus machine:
mkdir /sys/fs/cgroup/cpuset/test
cd /sys/fs/cgroup/cpuset/test
echo 1 > cpuset.cpus
echo 0 > cpuset.mems
sleep 100 & echo $! > tasks
sleep 100 & echo $! > tasks
echo 0 > /sys/bus/cpu/devices/cpu1/online

backtrace:
BUG: unable to handle kernel NULL pointer dereference at 0000000000000cf8
IP: cpuset_can_attach+0x2f/0x140
Call Trace:
 ? cpuset_attach+0x30f/0x3d0
 cgroup_migrate_execute+0x71/0x3c0
 cgroup_migrate+0x75/0x80
 cgroup_transfer_tasks+0x1b2/0x230
 cpuset_hotplug_workfn+0xa7d/0xce0
 ? finish_task_switch+0x79/0x240
 process_one_work+0x149/0x360
 worker_thread+0x4d/0x3c0

Signed-off-by: Shu Wang <shuwang@redhat.com>
---
 kernel/cgroup/cgroup-v1.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 024085daab1a..165734573b5e 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -129,6 +129,12 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
 		css_task_iter_end(&it);
 
 		if (task) {
+			/*
+			 * Reset csets to src_cets, as cgroup_migrate assumes
+			 * csets is pointing to src_csets.
+			 */
+			mgctx.tset.csets = &mgctx.tset.src_csets;
+
 			ret = cgroup_migrate(task, false, &mgctx);
 			if (!ret)
 				trace_cgroup_transfer_tasks(to, task, false);
-- 
2.13.5