RE: [Linux-ia64] Re: web page on O(1) scheduler

[Davide Libenzi]

On Fri, 23 May 2003, Davide Libenzi wrote:

> You need a write memory barrier even on the unlock. Consider this :
>
> 	spinlock = 1;
> 	...
> 	protected_resource = NEWVAL;
> 	spinlock = 0;
>
> ( where spinlock = 0/1 strip down, but do not lose the concept, the lock
> operation ). If a CPU reorder those writes, another CPU might see the lock
> drop before the protected resource assignment. And this is usually bad
> for obvious reasons.

David made me notice about a brain misfire here. You need protection even
for loads crossing the unlock. For the same obvious reasons :) Yes, a
realease barrier will be sufficent for ia64, while you'll need an mfence
on P4.

- Davide