Date: Mon, 28 Nov 2005 21:31:29 +0530
From: Dipankar Sarma <dipankar@in.ibm.com>
To: Andi Kleen <ak@suse.de>
Cc: akpm@osdl.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Allow lockless traversal of notifier lists
Message-ID: <20051128160129.GA8478@in.ibm.com>
Reply-To: dipankar@in.ibm.com
References: <20051128133757.GQ20775@brahms.suse.de>
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On Mon, Nov 28, 2005 at 02:37:57PM +0100, Andi Kleen wrote:
> 
> As discussed in other thread.
> 
> Just needed an additional write barrier, so that a parallel
> running lockup can never see inconsistent state. As long as there
> is no unregistration or the unregistration is done using
> locking or RCU in the caller they should be ok now.
> 
> This only makes a difference on non i386/x86-64 architectures.
> x86 was already ok because it never reorders writes.
> 
>   *
>   *	Currently always returns zero.
>   */
> @@ -116,6 +119,7 @@
>  		list= &((*list)->next);
>  	}
>  	n->next = *list;
> +	wmb();
>  	*list=n;
>  	write_unlock(&notifier_lock);

Shouldn't this be smp_wmb() ?

Also, not all archs have strong ordering for data dependent reads.
So, you would probably need an smp_read_barrier_depends() between
the load of the pointer and actual dereferencing.

Thanks
Dipankar
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