ceph: fix mounting same fs multiple times

Now __ceph_open_session() only accepts closed client. An opened
client will tigger BUG_ON().
Signed-off-by: NYan, Zheng <zyan@redhat.com>

fs/ceph/super.c

@@ -793,22 +793,20 @@ static struct dentry *ceph_real_mount(struct ceph_fs_client *fsc,
 	struct dentry *root;
 	int first = 0;   /* first vfsmount for this super_block */
 
-	dout("mount start\n");
+	dout("mount start %p\n", fsc);
 	mutex_lock(&fsc->client->mount_mutex);
 
-	err = __ceph_open_session(fsc->client, started);
-	if (err < 0)
-		goto out;
+	if (!fsc->sb->s_root) {
+		err = __ceph_open_session(fsc->client, started);
+		if (err < 0)
+			goto out;
 
-	dout("mount opening root\n");
-	root = open_root_dentry(fsc, "", started);
-	if (IS_ERR(root)) {
-		err = PTR_ERR(root);
-		goto out;
-	}
-	if (fsc->sb->s_root) {
-		dput(root);
-	} else {
+		dout("mount opening root\n");
+		root = open_root_dentry(fsc, "", started);
+		if (IS_ERR(root)) {
+			err = PTR_ERR(root);
+			goto out;
+		}
 		fsc->sb->s_root = root;
 		first = 1;
 
@@ -818,6 +816,7 @@ static struct dentry *ceph_real_mount(struct ceph_fs_client *fsc,
 	}
 
 	if (path[0] == 0) {
+		root = fsc->sb->s_root;
 		dget(root);
 	} else {
 		dout("mount opening base mountpoint\n");
@@ -833,16 +832,14 @@ static struct dentry *ceph_real_mount(struct ceph_fs_client *fsc,
 	mutex_unlock(&fsc->client->mount_mutex);
 	return root;
 
-out:
-	mutex_unlock(&fsc->client->mount_mutex);
-	return ERR_PTR(err);
-
 fail:
 	if (first) {
 		dput(fsc->sb->s_root);
 		fsc->sb->s_root = NULL;
 	}
-	goto out;
+out:
+	mutex_unlock(&fsc->client->mount_mutex);
+	return ERR_PTR(err);
 }
 
 static int ceph_set_super(struct super_block *s, void *data)