Looks like there is a race between set_cpus_allowed()
and move_task_off_dead_cpu().
__migrate_task() doesn't report any err code, so
task can be left on its runqueue if its cpus_allowed mask
changed so that dest_cpu is not longer a possible target.
Also, chaning cpus_allowed mask requires rq->lock being held.

Signed-Off-By: Kirill Korotaev <dev@openvz.org>

Kirill
P.S. against 2.6.17-mm1