[PATCH 11/13] vfs: Implement mount_super_once

[Eric W. Biederman]

The devpts filesystem has a notion of a system or primary instance of
devpts.  To retain the notion of a primary system instance of devpts
the code needs a way to allow userspace to mount the internally
mounted instance of devpts when it is not currently mounted by
userspace.  The new helper mount_super_once allows that.

Ideally mount_super_once would ignore still referenced lazy unmounts,
but in testing I was not able to find an existing distribution that
cared.  Since no one actually cares I did not try and solve the
formidable challenges of adding a test to ignore still referenced lazy
mounts in a race free way.

Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
 fs/super.c         | 12 ++++++++++++
 include/linux/fs.h |  2 ++
 2 files changed, 14 insertions(+)

diff --git a/fs/super.c b/fs/super.c
index 74914b1bae70..98a569412036 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1101,6 +1101,18 @@ struct dentry *mount_single(struct file_system_type *fs_type,
 }
 EXPORT_SYMBOL(mount_single);
 
+struct dentry *mount_super_once(struct super_block *sb, int flags, void *data)
+{
+	/* Allow mounting the specified superblock by userspace if there
+	 * are not any existing userspace mounts of it.
+	 */
+	if (atomic_cmpxchg(&sb->s_active, 1, 2) != 1)
+		return ERR_PTR(-EBUSY);
+	down_write(&sb->s_umount);
+	do_remount_sb(sb, flags, data, 0);
+	return dget(sb->s_root);
+}
+
 struct dentry *
 mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
 {
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 02a980bfad5c..515f874a6907 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2002,6 +2002,8 @@ extern struct dentry *mount_single(struct file_system_type *fs_type,
 extern struct dentry *mount_nodev(struct file_system_type *fs_type,
 	int flags, void *data,
 	int (*fill_super)(struct super_block *, void *, int));
+extern struct dentry *mount_super_once(struct super_block *sb,
+	int flags, void *data);
 extern struct dentry *mount_subtree(struct vfsmount *mnt, const char *path);
 void generic_shutdown_super(struct super_block *sb);
 void kill_block_super(struct super_block *sb);
-- 
2.6.3