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

[David Howells <dhowells@redhat.com>]

linux@horizon.com wrote:

> > Can be turned into:
> > 
> > 	1,C,A	x = LL()
> > 	1,C,A	x |= 2;
> > 	1,C,A	SC(3) [success]
> > 	3,C,A	...
> 
> ... which can be turned back into
> 
>  	1,C,A	x = load()
>  	1,C,A	x' = x | 2;
>  	1,C,A	cmpxchg(x,x') [success]
>  	3,C,A	...

Which would be totally pointless.

If you have LL/SC, then the odds are you _don't_ have CMPXCHG, and that
CMPXCHG is implemented using LL/SC, so what you end up with is:


 	1,C,A	x = load()
 	1,C,A	x' = x | 2;
	1,C,A	y = LL()
	1,C,A	if (y == x)
	1,X,A		SC(x');
 	3,C,A	...

David
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