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

[Matthew Wilcox <matthew@wil.cx>]

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.
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