Define get_init_pts_sb()

See comments in the function header for details. The new interface will
be used in a follow-on patch.

Changelog [v2]:
	[Dave Hansen] Replace get_sb_ref() in fs/super.c with get_init_pts_sb()
	and make the new interface private to devpts
Signed-off-by: NSukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: NAlan Cox <alan@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/devpts/inode.c

@@ -305,10 +305,63 @@ devpts_fill_super(struct super_block *s, void *data, int silent)
 	return -ENOMEM;
 }
 
+static int compare_init_pts_sb(struct super_block *s, void *p)
+{
+	if (devpts_mnt)
+		return devpts_mnt->mnt_sb == s;
+
+	return 0;
+}
+
+/*
+ * get_init_pts_sb()
+ *
+ *     This interface is needed to support multiple namespace semantics in
+ *     devpts while preserving backward compatibility of the current 'single-
+ *     namespace' semantics. i.e all mounts of devpts without the 'newinstance'
+ *     mount option should bind to the initial kernel mount, like
+ *     get_sb_single().
+ *
+ *     Mounts with 'newinstance' option create a new private namespace.
+ *
+ *     But for single-mount semantics, devpts cannot use get_sb_single(),
+ *     because get_sb_single()/sget() find and use the super-block from
+ *     the most recent mount of devpts. But that recent mount may be a
+ *     'newinstance' mount and get_sb_single() would pick the newinstance
+ *     super-block instead of the initial super-block.
+ *
+ *     This interface is identical to get_sb_single() except that it
+ *     consistently selects the 'single-namespace' superblock even in the
+ *     presence of the private namespace (i.e 'newinstance') super-blocks.
+ */
+static int get_init_pts_sb(struct file_system_type *fs_type, int flags,
+		void *data, struct vfsmount *mnt)
+{
+        struct super_block *s;
+        int error;
+
+        s = sget(fs_type, compare_init_pts_sb, set_anon_super, NULL);
+        if (IS_ERR(s))
+                return PTR_ERR(s);
+
+        if (!s->s_root) {
+                s->s_flags = flags;
+                error = devpts_fill_super(s, data, flags & MS_SILENT ? 1 : 0);
+                if (error) {
+                        up_write(&s->s_umount);
+                        deactivate_super(s);
+                        return error;
+                }
+                s->s_flags |= MS_ACTIVE;
+        }
+        do_remount_sb(s, flags, data, 0);
+        return simple_set_mnt(mnt, s);
+}
+
 static int devpts_get_sb(struct file_system_type *fs_type,
 	int flags, const char *dev_name, void *data, struct vfsmount *mnt)
 {
-	return get_sb_single(fs_type, flags, data, devpts_fill_super, mnt);
+	return get_init_pts_sb(fs_type, flags, data, mnt);
 }
 
 static void devpts_kill_sb(struct super_block *sb)