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, 33 insertions, 8 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index d94ccd6ddafd..bfcb701f9490 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1925,15 +1925,14 @@ static int do_new_mount(struct path *path, char *type, int flags,
         if (IS_ERR(mnt))
                 return PTR_ERR(mnt);
 
-        return do_add_mount(mnt, path, mnt_flags, NULL);
+        return do_add_mount(mnt, path, mnt_flags);
 }
 
 /*
  * add a mount into a namespace's mount tree
- * - provide the option of adding the new mount to an expiration list
+ * - this unconditionally eats one of the caller's references to newmnt.
  */
-int do_add_mount(struct vfsmount *newmnt, struct path *path,
-                 int mnt_flags, struct list_head *fslist)
+int do_add_mount(struct vfsmount *newmnt, struct path *path, int mnt_flags)
 {
         int err;
 
@@ -1963,9 +1962,6 @@ int do_add_mount(struct vfsmount *newmnt, struct path *path,
         if ((err = graft_tree(newmnt, path)))
                 goto unlock;
 
-        if (fslist) /* add to the specified expiration list */
-                list_add_tail(&newmnt->mnt_expire, fslist);
-
         up_write(&namespace_sem);
         return 0;
 
@@ -1975,7 +1971,36 @@ unlock:
         return err;
 }
 
-EXPORT_SYMBOL_GPL(do_add_mount);
+/**
+ * mnt_set_expiry - Put a mount on an expiration list
+ * @mnt: The mount to list.
+ * @expiry_list: The list to add the mount to.
+ */
+void mnt_set_expiry(struct vfsmount *mnt, struct list_head *expiry_list)
+{
+        down_write(&namespace_sem);
+        br_write_lock(vfsmount_lock);
+
+        list_add_tail(&mnt->mnt_expire, expiry_list);
+
+        br_write_unlock(vfsmount_lock);
+        up_write(&namespace_sem);
+}
+EXPORT_SYMBOL(mnt_set_expiry);
+
+/*
+ * Remove a vfsmount from any expiration list it may be on
+ */
+void mnt_clear_expiry(struct vfsmount *mnt)
+{
+        if (!list_empty(&mnt->mnt_expire)) {
+                down_write(&namespace_sem);
+                br_write_lock(vfsmount_lock);
+                list_del_init(&mnt->mnt_expire);
+                br_write_unlock(vfsmount_lock);
+                up_write(&namespace_sem);
+        }
+}
 
 /*
  * process a list of expirable mountpoints with the intent of discarding any