MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Message-ID: <17431.14867.211423.851470@cargo.ozlabs.ibm.com>
Date: Wed, 15 Mar 2006 08:48:03 +1100
From: Paul Mackerras <paulus@samba.org>
To: David Howells <dhowells@redhat.com>
Cc: ebiederm@xmission.com (Eric W. Biederman), akpm@osdl.org,
       linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
       torvalds@osdl.org, mingo@redhat.com, alan@redhat.com,
       linuxppc64-dev@ozlabs.org
Subject: Re: [PATCH] Document Linux's memory barriers [try #4] 
In-Reply-To: <32068.1142371612@warthog.cambridge.redhat.com>
References: <m1veujy47r.fsf@ebiederm.dsl.xmission.com>
	<16835.1141936162@warthog.cambridge.redhat.com>
	<32068.1142371612@warthog.cambridge.redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 939
Lines: 24

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/