Messages in this thread | | | Date | Mon, 23 Jun 2008 10:48:46 +0200 | From | Nick Piggin <> | Subject | Re: [PATCH]rcu,inet,fib_trie,route,radix-tree,DECnet,mac80211: fix meaningless rcu_dereference(local_var) |
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On Mon, Jun 23, 2008 at 01:35:52PM +0800, Lai Jiangshan wrote: > 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?
It is "meaningless" in that it isn't being applied as the API is supposed to, however it does provide the barrier that's required. I guess read barrier depends could just be used instead, although I like the self commenting nature of the rcu_dereference, even if it is not quite applied correctly, the reader can easily see the intention.
I *think* it should even do the right thing WRT the access_once macro here, and cause node not to be reloaded from source, but I could be wrong on that.
> 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.
So it doesn't help me ;)
I'm not sure what the best way to go is, but I would ask Paul for ideas if he's not too busy.
In reality, the barriers probably don't matter much (but I'd really love to have an Alpha to test it on :)), but I still try to avoid them as much as possible. For the radix-tree as used by pagecache, it is a completely usual operation to lookup non existing elements, so we are talking about a fastpath of sorts...
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