drivers/block/brd.c: fix brd_lookup_page() race

The index on the page must be set before it is inserted in the radix
tree.  Otherwise there is a small race which can occur during lookup
where the page can be found with the incorrect index.  This will trigger
the BUG_ON() in brd_lookup_page().

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reported-by: Chris Wedgwood <cw@f00f.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/drivers/block/brd.c b/drivers/block/brd.c
index f1a29f8e9d33..9bf4371755f2 100644
--- a/drivers/block/brd.c
+++ b/drivers/block/brd.c
@@ -117,13 +117,13 @@ static struct page *brd_insert_page(struct brd_device *brd, sector_t sector)
 
         spin_lock(&brd->brd_lock);
         idx = sector >> PAGE_SECTORS_SHIFT;
+        page->index = idx;
         if (radix_tree_insert(&brd->brd_pages, idx, page)) {
                 __free_page(page);
                 page = radix_tree_lookup(&brd->brd_pages, idx);
                 BUG_ON(!page);
                 BUG_ON(page->index != idx);
-        } else
-                page->index = idx;
+        }
         spin_unlock(&brd->brd_lock);
 
         radix_tree_preload_end();