[PATCH 3.10 48/54] radix-tree: fix race in gang lookup

Greg Kroah-Hartman
2016-02-24T03:54:51Z

From: Matthew Wilcox <willy@linux.intel.com>

commit 46437f9a554fbe3e110580ca08ab703b59f2f95a upstream.

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@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
include/linux/radix-tree.h | 16 ++++++++++++++++
lib/radix-tree.c | 12 ++++++++++--
2 files changed, 26 insertions(+), 2 deletions(-)

--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -322,6 +322,22 @@ void **radix_tree_next_chunk(struct radi
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
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -1015,9 +1015,13 @@ radix_tree_gang_lookup(struct radix_tree
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;
}
@@ -1094,9 +1098,13 @@ radix_tree_gang_lookup_tag(struct radix_
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;
}