x86/reboot: Use NMI to assist in shutting down if IRQ fails

For v3.3, I added code to use the NMI to stop other cpus in the
panic case.  The idea was to make sure all cpus on the system
were definitely halted to help serialize the panic path to
execute the rest of the code on a single cpu.

The main problem it was trying to solve was how to stop a cpu
that was spinning with its irqs disabled.  A IPI irq would be
stuck and couldn't get in there, but an NMI could.

Things were great until we had another conversation about some
pstore changes.  Because some of the backend pstore still uses
spinlocks to protect the device access, things could get ugly if
a panic happened and we were stuck spinning on a lock.

Now with the NMI shutting down cpus, we could assume no other
cpus were running and just bust the spin lock and proceed.

The counter argument was, well if you do that the backend could
be in a screwed up state and you might not be able to save
anything as a result. If we could have just given the cpu a
little more time to finish things, we could have grabbed the
spin lock cleanly and everything would have been fine.

Well, how do give a cpu a 'little more time' in the panic case?
For the most part you can't without spinning on the lock and
even in that case, how long do you spin for?

So instead of making it ugly in the pstore code, just mimic the
idea that stop_machine had, which is block on an IRQ IPI until
the remote cpu has re-enabled interrupts and left the critical
region.  Which is what happens now using REBOOT_IRQ.

Then leave the NMI case for those cpus that are truly stuck
after a short time.  This leaves the current behaviour alone and
just handle a corner case.  Most systems should never have to
enter the NMI code and if they do, print out a message in case
the NMI itself causes another issue.

Signed-off-by: Don Zickus <dzickus@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1336761675-24296-3-git-send-email-dzickus@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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