Unexport do_add_mount() and add in follow_automount(), not ->d_automount()

Unexport do_add_mount() and make ->d_automount() return the vfsmount to be
added rather than calling do_add_mount() itself.  follow_automount() will then
do the addition.

This slightly complicates things as ->d_automount() normally wants to add the
new vfsmount to an expiration list and start an expiration timer.  The problem
with that is that the vfsmount will be deleted if it has a refcount of 1 and
the timer will not repeat if the expiration list is empty.

To this end, we require the vfsmount to be returned from d_automount() with a
refcount of (at least) 2.  One of these refs will be dropped unconditionally.
In addition, follow_automount() must get a 3rd ref around the call to
do_add_mount() lest it eat a ref and return an error, leaving the mount we
have open to being expired as we would otherwise have only 1 ref on it.

d_automount() should also add the the vfsmount to the expiration list (by
calling mnt_set_expiry()) and start the expiration timer before returning, if
this mechanism is to be used.  The vfsmount will be unlinked from the
expiration list by follow_automount() if do_add_mount() fails.

This patch also fixes the call to do_add_mount() for AFS to propagate the mount
flags from the parent vfsmount.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 6 insertions, 19 deletions

diff --git a/fs/afs/mntpt.c b/fs/afs/mntpt.c
index d23b2e344a7..aa59184151d 100644
--- a/fs/afs/mntpt.c
+++ b/fs/afs/mntpt.c
@@ -241,7 +241,6 @@ error_no_devname:
 struct vfsmount *afs_d_automount(struct path *path)
 {
 	struct vfsmount *newmnt;
-	int err;
 
 	_enter("{%s,%s}", path->mnt->mnt_devname, path->dentry->d_name.name);
 
@@ -249,24 +248,12 @@ struct vfsmount *afs_d_automount(struct path *path)
 	if (IS_ERR(newmnt))
 		return newmnt;
 
-	mntget(newmnt);
-	err = do_add_mount(newmnt, path, MNT_SHRINKABLE, &afs_vfsmounts);
-	switch (err) {
-	case 0:
-		queue_delayed_work(afs_wq, &afs_mntpt_expiry_timer,
-				   afs_mntpt_expiry_timeout * HZ);
-		_leave(" = %p {%s}", newmnt, newmnt->mnt_devname);
-		return newmnt;
-	case -EBUSY:
-		/* someone else made a mount here whilst we were busy */
-		mntput(newmnt);
-		_leave(" = NULL [EBUSY]");
-		return NULL;
-	default:
-		mntput(newmnt);
-		_leave(" = %d", err);
-		return ERR_PTR(err);
-	}
+	mntget(newmnt); /* prevent immediate expiration */
+	mnt_set_expiry(newmnt, &afs_vfsmounts);
+	queue_delayed_work(afs_wq, &afs_mntpt_expiry_timer,
+			   afs_mntpt_expiry_timeout * HZ);
+	_leave(" = %p {%s}", newmnt, newmnt->mnt_devname);
+	return newmnt;
 }
 
 /*