[lug] more spinlock info

[D. Stimits]

For the person asking about spinlocks tonight on the Sparc SMP machine,
there is one addition I might add about spinlocks. While a regular
hardware or software interrupt won't allow a cpu in a spinlock to
context switch or go to an interrupt handler, a non-maskable interrupt
will (nmi). I assume Sparc's have an nmi? If not, it could account for
someone using fewer, or even zero, spinlocks in the Sparc smp
kernel...just a guess though. Watchdog timer boards typically use the
nmi to reboot if it detects failure on x86, and without it, a lockup due
to deadlock at a spinlock on all cpu's would cause reboot failure. Does
the Sparc cpu have an nmi?

D. Stimits, stimits at idcomm.com