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" <ebied...@xmission.com>
---
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
