Patch "radix-tree: fix race in gang lookup" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

radix-tree: fix race in gang lookup

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
radix-tree-fix-race-in-gang-lookup.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Matthew Wilcox <[email protected]>
Date: Tue, 2 Feb 2016 16:57:52 -0800
Subject: radix-tree: fix race in gang lookup

From: Matthew Wilcox <[email protected]>

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.

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

@@ -370,6 +370,22 @@ 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

@@ -1019,9 +1019,13 @@ 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;
 	}

@@ -1098,9 +1102,13 @@ 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;
 	}

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/radix-tree-fix-oops-after-radix_tree_iter_retry.patch queue-4.4/drivers-hwspinlock-fix-race-between-radix-tree-insertion-and-lookup.patch queue-4.4/radix-tree-fix-race-in-gang-lookup.patch queue-4.4/devm_memremap_release-fix-memremap-d-addr-handling.patch