A fix for the CPU hotplug and cpusets interaction: cpusets delegate the hotplug work to a workqueue to prevent a lock order inversion vs. the CPU hotplug lock. The work is not flushed before the hotplug operation returns which creates user visible inconsistent state. Prevent this by flushing the work after dropping CPU hotplug lock and before releasing the outer mutex which serializes the CPU hotplug related sysfs interface operations.