lkml.org 
[lkml]   [2006]   [Mar]   [14]   [last100]   RSS Feed
Views: [more markup]  [less markup]  [headers]  [forward] 
 
Messages in this thread
/
DateWed, 15 Mar 2006 08:48:03 +1100
FromPaul Mackerras <>
SubjectRe: [PATCH] Document Linux's memory barriers [try #4]
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.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2006-03-14 21:50    [from the cache]
©2003-2008