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SubjectRe: [PATCH]rcu,inet,fib_trie,route,radix-tree,DECnet,mac80211: fix meaningless rcu_dereference(local_var)
Add CC: Linus Torvalds

Nick Piggin wrote:
> 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.
>
>
>
This(http://lkml.org/lkml/2008/4/20/217) shows why rcu_dereference(local_var)
is meaningless. And why not use smp_read_barrier_depends() here?

I guessed somebody use rcu_dereference(local_var) in if-statements to avoid the
barrier when possible, and I made this patch(http://lkml.org/lkml/2008/6/21/29),
but it is incorrect.



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