Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it

Matthew Wilcox
2006-12-06T21:36:29Z

On Wed, Dec 06, 2006 at 07:58:20PM +0000, Russell King wrote:
> No. If you read what I said, you'll see that you can _cheaply_ use
> cmpxchg in a ll/sc based implementation. Take an atomic increment
> operation.
>
> do {
> old = load_locked(addr);
> } while (store_exclusive(old, old + 1, addr);

[...]

> Implementing ll/sc based accessor macros allows both ll/sc _and_ cmpxchg
> architectures to produce optimal code.
>
> Implementing an cmpxchg based accessor macro allows cmpxchg architectures
> to produce optimal code and ll/sc non-optimal code.

And for those of us with only load-and-zero, that's simply:

#define load_locked(addr) spin_lock(hash(addr)), *addr
#define store_exclusive(addr, old, new) \
*addr = new, spin_unlock(hash(addr)), 0

which is also optimal for us.