Re: [PATCH] Allow lockless traversal of notifier lists

[Dipankar Sarma]

On Tue, Nov 29, 2005 at 01:01:58AM +0100, Andi Kleen wrote:
> On Mon, Nov 28, 2005 at 11:12:03PM +0530, Dipankar Sarma wrote:
> 
> Ok third version, hopefully Dipankar proof now. 
> 

Not quite. I spoke without looking at the code of the whole
notifier_call_chain() function.

> + * 	against parallel traversals.
>   *
>   *	Returns zero on success, or %-ENOENT on failure.
>   */
> @@ -171,10 +177,12 @@
>  int notifier_call_chain(struct notifier_block **n, unsigned long val, void *v)
>  {
>  	int ret=NOTIFY_DONE;
> -	struct notifier_block *nb = *n;
> -
> +	struct notifier_block *nb;
> +	smp_read_barrier_depends();
> +	nb = *n;
>  	while(nb)
>  	{
> +		smp_read_barrier_depends();
>  		ret=nb->notifier_call(nb,val,v);
>  		if(ret&NOTIFY_STOP_MASK)
>  		{

Looking at the full code, it seems to me that we dereference
the first notifier block only inside the while(nb) loop.
That means the smp_read_barrier_depends() in the while(nb)
loop should be sufficient - IOW, the previous version of
the patch with one smp_read_barrier_depends() was good. 
Sorry about the confusion.

Thanks
Dipankar
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