[PATCH] Document Linux's memory barriers [try #4]

[Paul Mackerras]

David Howells writes:

> +	CPU 1		CPU 2		COMMENT
> +	===============	===============	=======================================
> +					a == 0, b == 1 and p == &a, q == &a
> +	b = 2;
> +	smp_wmb();			Make sure b is changed before p
> +	p = &b;		q = p;
> +			d = *q;
> +
> +then old data values may be used in the address calculation for the second
> +value, potentially resulting in q == &b and d == 0 being seen, which is never
> +correct.  What is required is a data dependency memory barrier:

No, that's not the problem.  The problem is that you can get q == &b
and d == 1, believe it or not.  That is, you can see the new value of
the pointer but the old value of the thing pointed to.

Paul.