Btrfs: Fix a crash when mounting a subvolume

We should drop dentry before deactivating the superblock, otherwise
we can hit this bug:

BUG: Dentry f349a690{i=100,n=/} still in use (1) [unmount of btrfs loop1]
...

Steps to reproduce the bug:

  # mount /dev/loop1 /mnt
  # mkdir save
  # btrfs subvolume snapshot /mnt save/snap1
  # umount /mnt
  # mount -o subvol=save/snap1 /dev/loop1 /mnt
  (crash)

Reported-by: Michael Niederle <mniederle@gmx.at>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 47bf67cbe6bf..61bd79abb805 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -685,9 +685,9 @@ static int btrfs_get_sb(struct file_system_type *fs_type, int flags,
                 mutex_unlock(&root->d_inode->i_mutex);
 
                 if (IS_ERR(new_root)) {
+                        dput(root);
                         deactivate_locked_super(s);
                         error = PTR_ERR(new_root);
-                        dput(root);
                         goto error_free_subvol_name;
                 }
                 if (!new_root->d_inode) {