ceph: fix dentry rehashing on virtual .snap dir

If a lookup fails on the magic .snap directory, we bind it to a magic
snap directory inode in ceph_lookup_finish().  That code assumes the dentry
is unhashed, but a recent server-side change started returning NULL leases
on lookup failure, causing the .snap dentry to be hashed and NULL by
ceph_fill_trace().

This causes dentry hash chain corruption, or a dies when d_rehash()
includes
	BUG_ON(!d_unhashed(entry));

So, avoid processing the NULL dentry lease if it the dentry matches the
snapdir name in ceph_fill_trace().  That allows the lookup completion to
properly bind it to the snapdir inode.  BUG there if dentry is hashed to
be sure.
Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/dir.c

@@ -488,6 +488,7 @@ struct dentry *ceph_finish_lookup(struct ceph_mds_request *req,
 		struct inode *inode = ceph_get_snapdir(parent);
 		dout("ENOENT on snapdir %p '%.*s', linking to snapdir %p\n",
 		     dentry, dentry->d_name.len, dentry->d_name.name, inode);
+		BUG_ON(!d_unhashed(dentry));
 		d_add(dentry, inode);
 		err = 0;
 	}

fs/ceph/inode.c

@@ -886,6 +886,7 @@ int ceph_fill_trace(struct super_block *sb, struct ceph_mds_request *req,
 	struct inode *in = NULL;
 	struct ceph_mds_reply_inode *ininfo;
 	struct ceph_vino vino;
+	struct ceph_client *client = ceph_sb_to_client(sb);
 	int i = 0;
 	int err = 0;
 
@@ -949,7 +950,14 @@ int ceph_fill_trace(struct super_block *sb, struct ceph_mds_request *req,
 			return err;
 	}
 
-	if (rinfo->head->is_dentry && !req->r_aborted) {
+	/*
+	 * ignore null lease/binding on snapdir ENOENT, or else we
+	 * will have trouble splicing in the virtual snapdir later
+	 */
+	if (rinfo->head->is_dentry && !req->r_aborted &&
+	    (rinfo->head->is_target || strncmp(req->r_dentry->d_name.name,
+					       client->mount_args->snapdir_name,
+					       req->r_dentry->d_name.len))) {
 		/*
 		 * lookup link rename   : null -> possibly existing inode
 		 * mknod symlink mkdir  : null -> new inode