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>

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();