softirq: Fix unplug deadlock

If ksoftirqd gets woken during hot-unplug, __thread_do_softirq() will
call pin_current_cpu() which will block on the held cpu_hotplug.lock.
Moving the offline check in __thread_do_softirq() before the
pin_current_cpu() call doesn't work, since the wakeup can happen
before we mark the cpu offline.

So here we have the ksoftirq thread stuck until hotplug finishes, but
then the ksoftirq CPU_DOWN notifier issues kthread_stop() which will
wait for the ksoftirq thread to go away -- while holding the hotplug
lock.

Sort this by delaying the kthread_stop() until CPU_POST_DEAD, which is
outside of the cpu_hotplug.lock, but still serialized by the
cpu_add_remove_lock.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: rostedt <rostedt@goodmis.org>
Cc: Clark Williams <williams@redhat.com>
Link: http://lkml.kernel.org/r/1317391156.12973.3.camel@twins
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

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