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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Lai Jiangshan <laijs@cn.fujitsu.com>
Subject: Re: [PATCH]rcu,inet,fib_trie,route,radix-tree,DECnet,mac80211: fix meaningless rcu_dereference(local_var)
Date: Mon, 23 Jun 2008 12:36:50 +1000
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Cc: Andrew Morton <akpm@linux-foundation.org>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Luis Carlos Cobo <luisca@cozybit.com>,
       Steve Whitehouse <SteveW@acm.org>,
       Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>, Nick Piggin <npiggin@suse.de>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Saturday 21 June 2008 19:54, Lai Jiangshan wrote:

> diff --git a/lib/radix-tree.c b/lib/radix-tree.c
> index 169a2f8..bfae4e2 100644
> --- a/lib/radix-tree.c
> +++ b/lib/radix-tree.c
> @@ -703,9 +703,9 @@ __lookup(struct radix_tree_node *slot, void **results,
> unsigned long index, for (i = index & RADIX_TREE_MAP_MASK; i <
> RADIX_TREE_MAP_SIZE; i++) { struct radix_tree_node *node;
>  		index++;
> -		node = slot->slots[i];
> +		node = rcu_dereference(slot->slots[i]);
>  		if (node) {
> -			results[nr_found++] = rcu_dereference(node);
> +			results[nr_found++] = node;
>  			if (nr_found == max_items)
>  				goto out;
>  		}
> @@ -815,7 +815,7 @@ __lookup_tag(struct radix_tree_node *slot, void
> **results, unsigned long index, index++;
>  				if (!tag_get(slot, tag, j))
>  					continue;
> -				node = slot->slots[j];
> +				node = rcu_dereference(slot->slots[j]);
>  				/*
>  				 * Even though the tag was found set, we need to
>  				 * recheck that we have a non-NULL node, because
> @@ -827,7 +827,6 @@ __lookup_tag(struct radix_tree_node *slot, void
> **results, unsigned long index, * rely on its value remaining the same).
>  				 */
>  				if (node) {
> -					node = rcu_dereference(node);
>  					results[nr_found++] = node;
>  					if (nr_found == max_items)
>  						goto out;

This was done like this IIRC to avoid the barrier when possible.