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

Paul Mackerras
2006-03-14T21:48:09Z

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.