radix-tree: fix oops after radix_tree_iter_retry

Helper radix_tree_iter_retry() resets next_index to the current index.
In following radix_tree_next_slot current chunk size becomes zero.  This
isn't checked and it tries to dereference null pointer in slot.

Tagged iterator is fine because retry happens only at slot 0 where tag
bitmask in iter->tags is filled with single bit.

Fixes: 46437f9 ("radix-tree: fix race in gang lookup")
Signed-off-by: Konstantin Khlebnikov <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Ohad Ben-Cohen <[email protected]>
Cc: Jeremiah Mahler <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: koct9i

include/linux/radix-tree.h

@@ -400,7 +400,7 @@ void **radix_tree_iter_retry(struct radix_tree_iter *iter)
  * @iter:	pointer to radix tree iterator
  * Returns:	current chunk size
  */
-static __always_inline unsigned
+static __always_inline long
 radix_tree_chunk_size(struct radix_tree_iter *iter)
 {
 	return iter->next_index - iter->index;
@@ -434,9 +434,9 @@ 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;
+		long size = radix_tree_chunk_size(iter);
 
-		while (size--) {
+		while (--size > 0) {
 			slot++;
 			iter->index++;
 			if (likely(*slot))