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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