From: Paul Mackerras <paulus@samba.org>

After I sent the recent patch to include/asm-ppc64/io.h which put stronger
barriers in the I/O accessor macros, Paul McKenney pointed out to me that a
writex/outx could still slide out from inside a spinlocked region.  This patch
makes the barriers a bit stronger so that this can't happen.  It means that we
need to use a sync instruction for wmb (a full "heavyweight" sync), since
drivers rely on wmb for ordering between writes to system memory and writes to
a device.

I have left smb_wmb() as a lighter-weight barrier that orders stores, and
doesn't impose an ordering between cacheable and non-cacheable accesses (the
amusingly-named eieio instruction).  I am assuming here that smp_wmb is only
used for ordering stores to system memory so that another cpu will see them in
order.  It can't be used for enforcing any ordering that a device will see,
because it is just a gcc barrier on UP.

This also changes the spinlock/rwlock unlock code to use lwsync ("light-weight
sync") rather than eieio, since eieio doesn't order loads, and we need to
ensure that loads stay inside the spinlocked region.