[parisc-linux] ldcw in __pthread_acquire

[LaMont Jones]

> I think LaMont Jones meant that the spinlock loop (in user space AND in kerne
> space) must implement a load word loop, and issue a load-and-clear word only
> when the lock word looks 'free', this is because the 'write' part of the
> load-and-clear word issue a cache broadcast transaction on the bus, so the
> loop should be on load-word followed by load-clear-word when it looks free,

That was one of the first solutions tried in HP-UX, and it resulted in
processor 4 not getting any time (3 wasn't much better), due to the way
that bus arbitration works (it favors one end of the bus.)

The current semaphore operations in the HP-UX kernel do not use ldcw: they
use stb and ldw in some interesting orders (which break when we get weak
ordering with IA64, but then we'll have a low-cost test-and-set.)

lamont