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Re: Slight tweak to percpu API

> Something about the current percpu API bothers me a little.  It  
> currently says that callers must disable preemption before calling  
> percpu_getptr().
> That seems cumbersome.  It seems to be the percpu API could do this  
> for you.  So I've changed the API like so:
> - percpu_getptr() is now called percpu_putref() and implicitly  
> disables preemption (via crit_enter()) when it is called.
> - Added percpu_putref() which implicitly reenables preemption (via  
> crit_exit()).
> I also updated the only current in-tree user of the percpu API ...  
> uipc_mbuf.c.
> Attached are the diffs.  Any objections?

i did this in the current way so that where preemption is known to be
disabled already (eg. interrupts) you can avoid unnecessary crit_enter/exit
overheads.  i don't object against the change.  just FYI.


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