Btrfs: Fix the unplug_io_fn to grab a consistent copy of page->mapping

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 12 insertions, 1 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 19c258d2c7e2..1281c393c7e6 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -936,14 +936,25 @@ void btrfs_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
         struct inode *inode;
         struct extent_map_tree *em_tree;
         struct extent_map *em;
+        struct address_space *mapping;
         u64 offset;
 
+        /* the generic O_DIRECT read code does this */
         if (!page) {
                 __unplug_io_fn(bdi, page);
                 return;
         }
 
-        inode = page->mapping->host;
+        /*
+         * page->mapping may change at any time.  Get a consistent copy
+         * and use that for everything below
+         */
+        smp_mb();
+        mapping = page->mapping;
+        if (!mapping)
+                return;
+
+        inode = mapping->host;
         offset = page_offset(page);
 
         em_tree = &BTRFS_I(inode)->extent_tree;