Messages in this thread | | | Date | Tue, 14 Mar 2006 09:37:04 +1100 | From | Nick Piggin <> | Subject | Re: [patch 1/3] radix tree: RCU lockless read-side |
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Balbir Singh wrote:
><snip> > >>But we should have already rcu_dereference()ed "slot", right >>(in the loop above this one)? That means we are now able to >>dereference it, and the data at the other end will be valid. >> >> > >Yes, but my confusion is about the following piece of code > ><begin code> > > for ( ; height > 1; height--) { > > for (i = (index >> shift) & RADIX_TREE_MAP_MASK ; > i < RADIX_TREE_MAP_SIZE; i++) { >- if (slot->slots[i] != NULL) >+ __s = rcu_dereference(slot->slots[i]); >+ if (__s != NULL) > break; > index &= ~((1UL << shift) - 1); > index += 1UL << shift; >@@ -531,14 +550,14 @@ __lookup(struct radix_tree_root *root, v > goto out; > > shift -= RADIX_TREE_MAP_SHIFT; >- slot = slot->slots[i]; >+ slot = __s; > } > > /* Bottom level: grab some items */ > for (i = index & RADIX_TREE_MAP_MASK; i < RADIX_TREE_MAP_SIZE; i++) { > index++; > if (slot->slots[i]) { >- results[nr_found++] = slot->slots[i]; >+ results[nr_found++] = &slot->slots[i]; > if (nr_found == max_items) > goto out; > } ><end code> > >In the for loop, lets say __s is *not* NULL, we break from the loop. >In the loop below >slot->slots[i] is derefenced without rcu, __s is not used. Is that not >inconsistent? > >
The "slots" member is an array, not an RCU assigned pointer. As such, after doing rcu_dereference(slot), you can access slot->slots[i] without further memory barriers I think?
But I agree that code now is a bit inconsistent. I've cleaned things up a bit in my tree now... but perhaps it is easier if you send a patch to show what you mean (because sometimes I'm a bit dense, I'm afraid).
Thanks, Nick
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