Re: [PATCH 1/19] MUTEX: Introduce simple mutex implementation

[Russell King]

On Sat, Dec 17, 2005 at 12:01:27AM +1100, Nick Piggin wrote:
> You were proposing a worse default, which is the reason I suggested it.

I'd like to qualify that.  "for architectures with native cmpxchg".

For general consumption (not specifically related to mutex stuff)...

For architectures with llsc, sequences stuch as:

	load
	modify
	cmpxchg

are inefficient because they have to be implemented as:

	load
	modify
	load
	compare
	store conditional

Now, if we consider using llsc as the basis of atomic operations:

	load
	modify
	store conditional

and for cmpxchg-based architectures:

	load
	modify
	cmpxchg

Notice that the cmpxchg-based case does _not_ get any worse - in fact
it's exactly identical.  Note, however, that the llsc case becomes
more efficient.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core
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