Bug in radix tree gang lookup?

From: Matthew Wilcox
Date: Fri Jan 22 2016 - 08:41:01 EST

I think there's a race in radix_tree_gang_lookup() (and
related functions). I was trying to understand why we need the
'indirect_to_ptr()' call here:

radix_tree_for_each_slot(slot, root, &iter, first_index) {
results[ret] = indirect_to_ptr(rcu_dereference_raw(*slot));
if (!results[ret])
if (++ret == max_items)

The slots returned are supposed to be leaf nodes, so why would they ever
have the indirect bit set?

The only two cases I can think of where we'd see a slot with the indirect
bit set is if we're calling radix_tree_gang_lookup() under the RCU read
lock and simultaneously growing / shrinking the tree. When the tree
transitions from height 0 to height 1, the 'slot' that was returned is now
an internal pointer, so simply knocking off the 'indirect_to_ptr()' bit
is the wrong thing to do; instead of returning a struct page pointer, we
return a pointer to a radix_tree_node, which isn't good. When shrinking
the tree from height 1 to height 0, we may end up looking at a pointer
in to-be-freed memory, but it's still a valid pointer to a struct page,
so I think we're OK in the shrink case.

The lockless page cache shows how to handle this correctly; when we
see an indirect bit come back in a slot, we should retry the lookup.
I think that's the right thing to do in this case, but I'd like someone
to check my reasoning before I propose a patch.