On Thu, Jan 28, 2016 at 12:17 AM, Matthew Wilcox
<matthew.r.wilcox@xxxxxxxxx> wrote:
> From: Matthew Wilcox <willy@xxxxxxxxxxxxxxx>
>
> If the indirect_ptr bit is set on a slot, that indicates we need to
> redo the lookup. Introduce a new function radix_tree_iter_retry()
> which forces the loop to retry the lookup by setting 'slot' to NULL and
> turning the iterator back to point at the problematic entry.
>
> This is a pretty rare problem to hit at the moment; the lookup has to
> race with a grow of the radix tree from a height of 0. The consequences
> of hitting this race are that gang lookup could return a pointer to a
> radix_tree_node instead of a pointer to whatever the user had inserted
> in the tree.
>
> Fixes: cebbd29e1c2f ("radix-tree: rewrite gang lookup using iterator")
> Signed-off-by: Matthew Wilcox <willy@xxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> include/linux/radix-tree.h | 16 ++++++++++++++++
> lib/radix-tree.c | 12 ++++++++++--
> 2 files changed, 26 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
> index f9a3da5bf892..db0ed595749b 100644
> --- a/include/linux/radix-tree.h
> +++ b/include/linux/radix-tree.h
> @@ -387,6 +387,22 @@ void **radix_tree_next_chunk(struct radix_tree_root *root,
> struct radix_tree_iter *iter, unsigned flags);
>
> /**
> + * radix_tree_iter_retry - retry this chunk of the iteration
> + * @iter: iterator state
> + *
> + * If we iterate over a tree protected only by the RCU lock, a race
> + * against deletion or creation may result in seeing a slot for which
> + * radix_tree_deref_retry() returns true. If so, call this function
> + * and continue the iteration.
> + */
> +static inline __must_check
> +void **radix_tree_iter_retry(struct radix_tree_iter *iter)
> +{
> + iter->next_index = iter->index;
> + return NULL;
> +}
> +
> +/**
> * radix_tree_chunk_size - get current chunk size
> *
> * @iter: pointer to radix tree iterator
> diff --git a/lib/radix-tree.c b/lib/radix-tree.c
> index a25f635dcc56..65422ac17114 100644
> --- a/lib/radix-tree.c
> +++ b/lib/radix-tree.c
> @@ -1105,9 +1105,13 @@ radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
> return 0;
>
> radix_tree_for_each_slot(slot, root, &iter, first_index) {
> - results[ret] = indirect_to_ptr(rcu_dereference_raw(*slot));
> + results[ret] = rcu_dereference_raw(*slot);
> if (!results[ret])
> continue;
> + if (radix_tree_is_indirect_ptr(results[ret])) {
> + slot = radix_tree_iter_retry(&iter);
> + continue;
> + }
> if (++ret == max_items)
> break;
> }
Looks like your fix doesn't work.
After radix_tree_iter_retry: radix_tree_for_each_slot will call
radix_tree_next_slot which isn't safe to call for NULL slot.
#define radix_tree_for_each_slot(slot, root, iter, start) \
for (slot = radix_tree_iter_init(iter, start) ; \
slot || (slot = radix_tree_next_chunk(root, iter, 0)) ; \
slot = radix_tree_next_slot(slot, iter, 0))
tagged iterator works becase restart happens only at root - tags
filled with single bit.
quick (untested) fix for that
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -457,9 +457,9 @@ radix_tree_next_slot(void **slot, struct
radix_tree_iter *iter, unsigned flags)
return slot + offset + 1;
}
} else {
- unsigned size = radix_tree_chunk_size(iter) - 1;
+ int size = radix_tree_chunk_size(iter) - 1;
- while (size--) {
+ while (size-- > 0) {
slot++;
iter->index++;
if (likely(*slot))
> @@ -1184,9 +1188,13 @@ radix_tree_gang_lookup_tag(struct radix_tree_root *root, void **results,
> return 0;
>
> radix_tree_for_each_tagged(slot, root, &iter, first_index, tag) {
> - results[ret] = indirect_to_ptr(rcu_dereference_raw(*slot));
> + results[ret] = rcu_dereference_raw(*slot);
> if (!results[ret])
> continue;
> + if (radix_tree_is_indirect_ptr(results[ret])) {
> + slot = radix_tree_iter_retry(&iter);
> + continue;
> + }
> if (++ret == max_items)
> break;
> }
> --
> 2.7.0.rc3
>
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