nfs: use IS_ROOT not DCACHE_DISCONNECTED

This check was added by Al Viro with
d9e80b7de91db05c1c4d2e5ebbfd70b3b3ba0e0f "nfs d_revalidate() is too
trigger-happy with d_drop()", with the explanation that we don't want to
remove the root of a disconnected tree, which will still be included on
the s_anon list.

But DCACHE_DISCONNECTED does *not* actually identify dentries that are
disconnected from the dentry tree or hashed on s_anon.  IS_ROOT() is the
way to do that.

Also add a comment from Al's commit to remind us why this check is
there.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index 02b0df769e2d..6cc51ae0638d 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -1139,7 +1139,13 @@ out_zap_parent:
         if (inode && S_ISDIR(inode->i_mode)) {
                 /* Purge readdir caches. */
                 nfs_zap_caches(inode);
-                if (dentry->d_flags & DCACHE_DISCONNECTED)
+                /*
+                 * We can't d_drop the root of a disconnected tree:
+                 * its d_hash is on the s_anon list and d_drop() would hide
+                 * it from shrink_dcache_for_unmount(), leading to busy
+                 * inodes on unmount and further oopses.
+                 */
+                if (IS_ROOT(dentry))
                         goto out_valid;
         }
         /* If we have submounts, don't unhash ! */