So why are we calling smp_send_stop from machine_halt?

We don't.

Looking more closely at the bug report the problem here
is that halt -p is called which triggers not a halt but
an attempt to power off.

machine_power_off calls machine_shutdown which calls smp_send_stop.

If pm_power_off is set we should never make it out machine_power_off
to the call of do_exit.  So pm_power_off must not be set in this case.
When pm_power_off is not set we expect machine_power_off to devolve
into machine_halt.

So how do we fix this?

Playing too much with smp_send_stop is dangerous because it
must also be safe to be called from panic.

It looks like the obviously correct fix is to only call
machine_shutdown when pm_power_off is defined.  Doing
that will make Andi's assumption about not scheduling
true and generally simplify what must be supported.

This turns machine_power_off into a noop like machine_halt
when pm_power_off is not defined.

If the expected behavior is that sys_reboot(LINUX_REBOOT_CMD_POWER_OFF)
becomes sys_reboot(LINUX_REBOOT_CMD_HALT) if pm_power_off is NULL
this is not quite a comprehensive fix as we pass a different parameter
to the reboot notifier and we set system_state to a different value
before calling device_shutdown().

Unfortunately any fix more comprehensive I can think of is not
obviously correct.  The core problem is that there is no architecture
independent way to detect if machine_power will become a noop, without
calling it.
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>