The new code maintains two variables 'current_spl_level' and
'hardware_spl_level'. Variable hardware_spl_level reflects actual
priority level at the hardware's point of view. hardware_spl_level is
always synchronized to hardware.
splraise() just increases current_spl_level. splx() sets
current_spl_level. If (and only if) hardware_spl_level and
current_spl_level is not same, splx() synchronizes interrupt mask
register and hardware_spl_level to current_spl_level.
In most case, splraise() raises current_spl_level and splx() restores
only current_spl_level.
When an interrupt occurs, hardware_spl_level and interrupt mask
register are synchronized to current_spl_level.
In this implementation, during a higher priority interrupt handler is
running, lower priority interrupts never cause intr_dispatch() to run.
It will avoid some race condition.
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