• Completely Remove the option STOP_NMI from the kernel. This option

has proven to have a good effect when entering KDB by using a NMI,
but it completely violates all the good rules about interrupts
disabled while holding a spinlock in other occasions. This can be the
cause of deadlocks on events where a normal IPI_STOP is expected.

• Adds an new IPI called IPI_STOP_HARD on all the supported architectures.

This IPI is responsible for sending a stop message among CPUs using a
privileged channel when disponible. In other cases it just does match a
normal IPI_STOP.
Right now the IPI_STOP_HARD functionality uses a NMI on ia32 and amd64
architectures, while on the other has a normal IPI_STOP effect. It is
responsibility of maintainers to eventually implement an hard stop
when necessary and possible.

• Use the new IPI facility in order to implement a new userend SMP kernel

function called stop_cpus_hard(). That is specular to stop_cpu() but
it does use the privileged channel for the stopping facility.

• Let KDB use the newly introduced function stop_cpus_hard() and leave

stop_cpus() for all the other cases

• Disable interrupts on CPU0 when starting the process of APs suspension.
• Style cleanup and comments adding

This patch should fix the reboot/shutdown deadlocks many users are
constantly reporting on mailing lists.

Please don't forget to update your config file with the STOP_NMI
option removal

Reviewed by: jhb
Tested by: pho, bz, rink
Approved by: re (kib)