cgroup: Reinit cgroup_taskset structure before cgroup_migrate_execute() returns

The cgroup_taskset structure within the larger cgroup_mgctx structure
is supposed to be used once and then discarded. That is not really the
case in the hotplug code path:

cpuset_hotplug_workfn()
 - cgroup_transfer_tasks()
   - cgroup_migrate()
     - cgroup_migrate_add_task()
     - cgroup_migrate_execute()

In this case, the cgroup_migrate() function is called multiple time
with the same cgroup_mgctx structure to transfer the tasks from
one cgroup to another one-by-one. The second time cgroup_migrate()
is called, the cgroup_taskset will be in an incorrect state and so
may cause the system to panic. For example,

  [  150.888410] Faulting instruction address: 0xc0000000001db648
  [  150.888414] Oops: Kernel access of bad area, sig: 11 [#1]
  [  150.888417] SMP NR_CPUS=2048
  [  150.888417] NUMA
  [  150.888419] pSeries
    :
  [  150.888545] NIP [c0000000001db648] cpuset_can_attach+0x58/0x1b0
  [  150.888548] LR [c0000000001db638] cpuset_can_attach+0x48/0x1b0
  [  150.888551] Call Trace:
  [  150.888554] [c0000005f65cb940] [c0000000001db638] cpuset_can_attach+0x48/0x1b 0 (unreliable)
  [  150.888559] [c0000005f65cb9a0] [c0000000001cff04] cgroup_migrate_execute+0xc4/0x4b0
  [  150.888563] [c0000005f65cba20] [c0000000001d7d14] cgroup_transfer_tasks+0x1d4/0x370
  [  150.888568] [c0000005f65cbb70] [c0000000001ddcb0] cpuset_hotplug_workfn+0x710/0x8f0
  [  150.888572] [c0000005f65cbc80] [c00000000012032c] process_one_work+0x1ac/0x4d0
  [  150.888576] [c0000005f65cbd20] [c0000000001206f8] worker_thread+0xa8/0x5b0
  [  150.888580] [c0000005f65cbdc0] [c0000000001293f8] kthread+0x168/0x1b0
  [  150.888584] [c0000005f65cbe30] [c00000000000b368] ret_from_kernel_thread+0x5c/0x74

To allow reuse of the cgroup_mgctx structure, some fields in that
structure are now re-initialized at the end of cgroup_migrate_execute()
function call so that the structure can be reused again in a later
iteration without causing problem.

This bug was introduced in the commit e595cd7069 ("group: track
migration context in cgroup_mgctx") in 4.11. This commit moves the
cgroup_taskset initialization out of cgroup_migrate(). The commit
10467270fb3 ("cgroup: don't call migration methods if there are no
tasks to migrate") helped, but did not completely resolve the problem.

Fixes: e595cd7069 ("group: track migration context in cgroup_mgctx")
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org # v4.11+

kernel/cgroup/cgroup.c

@@ -2311,6 +2311,14 @@ out_release_tset:
 		list_del_init(&cset->mg_node);
 	}
 	spin_unlock_irq(&css_set_lock);
+
+	/*
+	 * Re-initialize the cgroup_taskset structure in case it is reused
+	 * again in another cgroup_migrate_add_task()/cgroup_migrate_execute()
+	 * iteration.
+	 */
+	tset->nr_tasks = 0;
+	tset->csets    = &tset->src_csets;
 	return ret;
 }