NFS: Fix an Oops in NFS unmount

Ensure that the dummy 'root dentry' is invisible to d_find_alias(). If not,
then it may be spliced into the tree if a parent directory from the same
filesystem gets mounted at a later time.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/fs/nfs/getroot.c b/fs/nfs/getroot.c
index 0ee43843f4ec..e6242cdbaf91 100644
--- a/fs/nfs/getroot.c
+++ b/fs/nfs/getroot.c
@@ -57,6 +57,17 @@ static int nfs_superblock_set_dummy_root(struct super_block *sb, struct inode *i
                 }
                 /* Circumvent igrab(): we know the inode is not being freed */
                 atomic_inc(&inode->i_count);
+                /*
+                 * Ensure that this dentry is invisible to d_find_alias().
+                 * Otherwise, it may be spliced into the tree by
+                 * d_materialise_unique if a parent directory from the same
+                 * filesystem gets mounted at a later time.
+                 * This again causes shrink_dcache_for_umount_subtree() to
+                 * Oops, since the test for IS_ROOT() will fail.
+                 */
+                spin_lock(&dcache_lock);
+                list_del_init(&sb->s_root->d_alias);
+                spin_unlock(&dcache_lock);
         }
         return 0;
 }