PLEASE TEST: IPI deadlock avoidance patch

Hey folks,

If you have a reasonably fast i386 or amd64 multiprocessor and/or
hyperthreading machine and are experiencing reproducible hangs during -j
buildwords and other highly parallel operations, please try this patch:

http://people.freebsd.org/~dwhite/smp_rv_mtx.patch

This patch avoids a deadlock between the smp_rendezvous() mechanism and
TLB shootdowns via pmap by forcing them to share a mutex.  The patch has
been tested on several multiproc Xeon systems and a dual Opteron I have
here and is stable, but a couple of more eyeballs and varied loads
wouldn't hurt since this will be going into RELENG_5.

If there's anything glaringly wrong from a style point of view, let me
know too.

I plan on committing this late Monday or Tuesday, barring any serious
problems.

Thanks to alc for the nudge on the solution and rwatson for staying up
late and testing it on his hardware.

-- 
Doug White                    |  FreeBSD: The Power to Serve
dwhite_at_gumbysoft.com          |  www.FreeBSD.org