[PATCH] Change atomic operations to use fences for memory barriers

[John Baldwin]

The patch below changes the atomic operations on amd64 to use the cheaper 
fence instructions for memory barriers.  I'd like people to test it to see if 
1) it breaks things or not, and 2) if it impacts performance either in a good 
way or a bad way.  For this last I'm curious about both UP and SMP as my 
initial guess is that it will help on UP but might hurt on SMP.

-- 
John Baldwin <jhb at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org