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SubjectRe: Bug in radix tree gang lookup?
On Fri, 22 Jan 2016, Matthew Wilcox wrote:
>
> 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])
> continue;
> if (++ret == max_items)
> break;
> }
>
> 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.

I think you're right, in all you say above. And I think the last
paragraph of comment above the one-level-of-indirection-different
radix_tree_gang_lookup_slot() is making the same point, though its
language hasn't been updated for years (it ought to say something
like "radix_tree_deref_retry may require a retry").

Hugh

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