Okay, spinlocks are null ops when CONFIG_SMP and CONFIG_DEBUG_SPINLOCK
are both disabled, but you still have to disable interrupts, and that
slows things down, sometimes quite appreciably. It is, for example,
something I really want to avoid doing on FRV as it takes a *lot* of
cycles.
There was a USENIX paper a couple of decades ago that described how
to do a fast s/w disable of interrupts on machines where really disabling
interrupts was expensive. The rough gist was that the spl[1-7]()
functions would just set a flag in memory to hold the desired interrupt
mask. If an interrupt actually occurred when it was s/w blocked, the
handler would set a pending flag, and just rfi with interrupts disabled.
Then the splx() code checked to see whether there was a pending interrupt
and dealt with it if there was.
-Tony
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