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

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Date:	Wed, 6 Dec 2006 14:36:27 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Christoph Lameter <clameter@....com>,
	David Howells <dhowells@...hat.com>, torvalds@...l.org,
	akpm@...l.org, linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it

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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