convert devpts

Two kinds of objects there - ptmx and everything else (pty).  The former is
created on mount and kept until the fs shutdown; the latter get created
and removed by tty layer (the references are borrowed into tty->driver_data).
The reference to ptmx dentry is also kept, but we only ever use it to
find ptmx inode on remount.

* turn d_add() into d_make_persistent() + dput()  both in mknod_ptmx() and
in devpts_pty_new().

* turn dput() to d_make_discardable() in devpts_pty_kill().

* switch mknod_ptmx() to simple_{start,done}_creating().

* instead of storing in pts_fs_info a reference to ptmx dentry, store a reference
to its inode, seeing that this is what we use it for.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/devpts/inode.c

@@ -102,7 +102,7 @@ struct pts_fs_info {
 	struct ida allocated_ptys;
 	struct pts_mount_opts mount_opts;
 	struct super_block *sb;
-	struct dentry *ptmx_dentry;
+	struct inode *ptmx_inode; // borrowed
 };
 
 static inline struct pts_fs_info *DEVPTS_SB(struct super_block *sb)
@@ -259,7 +259,6 @@ static int devpts_parse_param(struct fs_context *fc, struct fs_parameter *param)
 static int mknod_ptmx(struct super_block *sb, struct fs_context *fc)
 {
 	int mode;
-	int rc = -ENOMEM;
 	struct dentry *dentry;
 	struct inode *inode;
 	struct dentry *root = sb->s_root;
@@ -268,18 +267,10 @@ static int mknod_ptmx(struct super_block *sb, struct fs_context *fc)
 	kuid_t ptmx_uid = current_fsuid();
 	kgid_t ptmx_gid = current_fsgid();
 
-	inode_lock(d_inode(root));
+	dentry = simple_start_creating(root, "ptmx");
-
+	if (IS_ERR(dentry)) {
-	/* If we have already created ptmx node, return */
-	if (fsi->ptmx_dentry) {
-		rc = 0;
-		goto out;
-	}
-
-	dentry = d_alloc_name(root, "ptmx");
-	if (!dentry) {
 		pr_err("Unable to alloc dentry for ptmx node\n");
-		goto out;
+		return PTR_ERR(dentry);
 	}
 
 	/*
@@ -287,9 +278,9 @@ static int mknod_ptmx(struct super_block *sb, struct fs_context *fc)
 	 */
 	inode = new_inode(sb);
 	if (!inode) {
+		simple_done_creating(dentry);
 		pr_err("Unable to alloc inode for ptmx node\n");
-		dput(dentry);
+		return -ENOMEM;
-		goto out;
 	}
 
 	inode->i_ino = 2;
@@ -299,23 +290,18 @@ static int mknod_ptmx(struct super_block *sb, struct fs_context *fc)
 	init_special_inode(inode, mode, MKDEV(TTYAUX_MAJOR, 2));
 	inode->i_uid = ptmx_uid;
 	inode->i_gid = ptmx_gid;
+	fsi->ptmx_inode = inode;
 
-	d_add(dentry, inode);
+	d_make_persistent(dentry, inode);
 
-	fsi->ptmx_dentry = dentry;
+	simple_done_creating(dentry);
-	rc = 0;
+
-out:
+	return 0;
-	inode_unlock(d_inode(root));
-	return rc;
 }
 
 static void update_ptmx_mode(struct pts_fs_info *fsi)
 {
-	struct inode *inode;
+	fsi->ptmx_inode->i_mode = S_IFCHR|fsi->mount_opts.ptmxmode;
-	if (fsi->ptmx_dentry) {
-		inode = d_inode(fsi->ptmx_dentry);
-		inode->i_mode = S_IFCHR|fsi->mount_opts.ptmxmode;
-	}
 }
 
 static int devpts_reconfigure(struct fs_context *fc)
@@ -461,7 +447,7 @@ static void devpts_kill_sb(struct super_block *sb)
 	if (fsi)
 		ida_destroy(&fsi->allocated_ptys);
 	kfree(fsi);
-	kill_litter_super(sb);
+	kill_anon_super(sb);
 }
 
 static struct file_system_type devpts_fs_type = {
@@ -534,16 +520,15 @@ struct dentry *devpts_pty_new(struct pts_fs_info *fsi, int index, void *priv)
 	sprintf(s, "%d", index);
 
 	dentry = d_alloc_name(root, s);
-	if (dentry) {
+	if (!dentry) {
-		dentry->d_fsdata = priv;
-		d_add(dentry, inode);
-		fsnotify_create(d_inode(root), dentry);
-	} else {
 		iput(inode);
-		dentry = ERR_PTR(-ENOMEM);
+		return ERR_PTR(-ENOMEM);
 	}
-
+	dentry->d_fsdata = priv;
-	return dentry;
+	d_make_persistent(dentry, inode);
+	fsnotify_create(d_inode(root), dentry);
+	dput(dentry);
+	return dentry; // borrowed
 }
 
 /**
@@ -573,7 +558,7 @@ void devpts_pty_kill(struct dentry *dentry)
 	drop_nlink(dentry->d_inode);
 	d_drop(dentry);
 	fsnotify_unlink(d_inode(dentry->d_parent), dentry);
-	dput(dentry);	/* d_alloc_name() in devpts_pty_new() */
+	d_make_discardable(dentry);
 }
 
 static int __init init_devpts_fs(void)