[RFC/PATCH] ARM: smp: Fix cpu_up() racing with sys_reboot

On Tue, Aug 21, 2012 at 09:03:49PM -0700, Stephen Boyd wrote:

Nothing stops a process from hotplugging in a CPU concurrently
with a sys_reboot() call. In such a situation we could have
ipi_cpu_stop() mark a cpu as 'offline' and _cpu_up() ignore the
fact that the CPU is not really offline and call the
CPU_UP_PREPARE notifier. When this happens stop_machine code will
complain that the cpu thread already exists and BUG_ON().

This puts us at odds with x86, which is a bad thing without first
investigating whether a generic solution which fixes all arches would
be more appropriate.

A better solution may be to mark those CPUs as being not-present,
which will prevent them being hot-plugged back.