[RFC PATCH] touch_mnt_namespace when the mount flags change

[Dan Williams <dan.j.williams@intel.com>]

Daemons that need to be launched while the rootfs is read-only can now
poll /proc/mounts to be notified when their O_RDWR requests may no
longer end in EROFS.

Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---

This change is prompted by mdmon which is a userspace metadata handler
for software-raid arrays.  It will use this notification to drop a
communication socket and pidfile on a raid-rootfs once it becomes
writable.

 fs/namespace.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 6e283c9..1bd5ba2 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1553,8 +1553,13 @@ static noinline int do_remount(struct nameidata *nd, int flags, int mnt_flags,
 	if (!err)
 		nd->path.mnt->mnt_flags = mnt_flags;
 	up_write(&sb->s_umount);
-	if (!err)
+	if (!err) {
 		security_sb_post_remount(nd->path.mnt, flags, data);
+
+		spin_lock(&vfsmount_lock);
+		touch_mnt_namespace(nd->path.mnt->mnt_ns);
+		spin_unlock(&vfsmount_lock);
+	}
 	return err;
 }