> is not held (which is the only case that matters, because otherwise,
> access would block on the console_lock currently), we have the following
> (for the x86 at least, the others are similar):
>
> spin_lock:
> "xchgb %b0,%1"
> "cmp $1, ..."
> "jz ..."
>
> Note that the jump is not taken, so it is one cycle.
The xchgb is a locked bus access so takes an awful lot longer, especially on
earlier cpus
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