[4.2.y-ckt stable] Patch "radix-tree: fix race in gang lookup" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    radix-tree: fix race in gang lookup

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt5.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 6e81aeb8201c2bdf6fc2eea452e0c26a7c50647e Mon Sep 17 00:00:00 2001
From: Matthew Wilcox <willy at linux.intel.com>
Date: Tue, 2 Feb 2016 16:57:52 -0800
Subject: radix-tree: fix race in gang lookup

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 at linux.intel.com>
Cc: Hugh Dickins <hughd at google.com>
Cc: Ohad Ben-Cohen <ohad at wizery.com>
Cc: Konstantin Khlebnikov <khlebnikov at openvz.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 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 33170db..1a2b227 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -370,6 +370,22 @@ 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 f9ebe1c..6608430 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -1019,9 +1019,13 @@ 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;
 	}
@@ -1098,9 +1102,13 @@ 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