[PATCH v2] kdump: Fix crash_kexec - smp_send_stop race in panic

[Luck, Tony]

> Instead of introducing the panic lock, as an alternative we could move
> smp_send_stop() to the beginning of panic(). Eric told me that the
> function is currently "insufficiently reliable" for that, but perhaps we
> could make it more reliable.

That's tough to do.  We are in panic because something went horribly
wrong somewhere in the kernel - so we can make few assumptions about
which subsystems are still working. In the worst case (for this example)
our panic was caused by a failure in the code that sends cross-processor
interrupts ... so calling that same code to stop the other cpus is
likely to run into the same problem - perhaps causing a nested panic.

So what looks like a good fix for some panic scenarios actually makes
others worse.

-Tony