Re: [PATCH]rcu,inet,fib_trie,route,radix-tree,DECnet,mac80211: fix meaningless rcu_dereference(local_var)

[Lai Jiangshan <laijs@cn.fujitsu.com>]

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.