[PATCH] shared mounts handling: umount

An unmount of a mount creates a umount event on the parent.  If the
parent is a shared mount, it gets propagated to all mounts in the peer
group.

Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

3 files changed, 126 insertions, 19 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 1487982dbc24..4b1af01c2fb4 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -86,31 +86,44 @@ void free_vfsmnt(struct vfsmount *mnt)
 }
 
 /*
- * Now, lookup_mnt increments the ref count before returning
- * the vfsmount struct.
+ * find the first or last mount at @dentry on vfsmount @mnt depending on
+ * @dir. If @dir is set return the first mount else return the last mount.
  */
-struct vfsmount *lookup_mnt(struct vfsmount *mnt, struct dentry *dentry)
+struct vfsmount *__lookup_mnt(struct vfsmount *mnt, struct dentry *dentry,
+                              int dir)
 {
         struct list_head *head = mount_hashtable + hash(mnt, dentry);
         struct list_head *tmp = head;
         struct vfsmount *p, *found = NULL;
 
-        spin_lock(&vfsmount_lock);
         for (;;) {
-                tmp = tmp->next;
+                tmp = dir ? tmp->next : tmp->prev;
                 p = NULL;
                 if (tmp == head)
                         break;
                 p = list_entry(tmp, struct vfsmount, mnt_hash);
                 if (p->mnt_parent == mnt && p->mnt_mountpoint == dentry) {
-                        found = mntget(p);
+                        found = p;
                         break;
                 }
         }
-        spin_unlock(&vfsmount_lock);
         return found;
 }
 
+/*
+ * lookup_mnt increments the ref count before returning
+ * the vfsmount struct.
+ */
+struct vfsmount *lookup_mnt(struct vfsmount *mnt, struct dentry *dentry)
+{
+        struct vfsmount *child_mnt;
+        spin_lock(&vfsmount_lock);
+        if ((child_mnt = __lookup_mnt(mnt, dentry, 1)))
+                mntget(child_mnt);
+        spin_unlock(&vfsmount_lock);
+        return child_mnt;
+}
+
 static inline int check_mnt(struct vfsmount *mnt)
 {
         return mnt->mnt_namespace == current->namespace;
@@ -404,9 +417,12 @@ EXPORT_SYMBOL(may_umount_tree);
  */
 int may_umount(struct vfsmount *mnt)
 {
-        if (atomic_read(&mnt->mnt_count) > 2)
-                return -EBUSY;
-        return 0;
+        int ret = 0;
+        spin_lock(&vfsmount_lock);
+        if (propagate_mount_busy(mnt, 2))
+                ret = -EBUSY;
+        spin_unlock(&vfsmount_lock);
+        return ret;
 }
 
 EXPORT_SYMBOL(may_umount);
@@ -433,7 +449,7 @@ void release_mounts(struct list_head *head)
         }
 }
 
-void umount_tree(struct vfsmount *mnt, struct list_head *kill)
+void umount_tree(struct vfsmount *mnt, int propagate, struct list_head *kill)
 {
         struct vfsmount *p;
 
@@ -442,6 +458,9 @@ void umount_tree(struct vfsmount *mnt, struct list_head *kill)
                 list_add(&p->mnt_hash, kill);
         }
 
+        if (propagate)
+                propagate_umount(kill);
+
         list_for_each_entry(p, kill, mnt_hash) {
                 list_del_init(&p->mnt_expire);
                 list_del_init(&p->mnt_list);
@@ -450,6 +469,7 @@ void umount_tree(struct vfsmount *mnt, struct list_head *kill)
                 list_del_init(&p->mnt_child);
                 if (p->mnt_parent != p)
                         mnt->mnt_mountpoint->d_mounted--;
+                change_mnt_propagation(p, MS_PRIVATE);
         }
 }
 
@@ -526,9 +546,9 @@ static int do_umount(struct vfsmount *mnt, int flags)
         event++;
 
         retval = -EBUSY;
-        if (atomic_read(&mnt->mnt_count) == 2 || flags & MNT_DETACH) {
+        if (flags & MNT_DETACH || !propagate_mount_busy(mnt, 2)) {
                 if (!list_empty(&mnt->mnt_list))
-                        umount_tree(mnt, &umount_list);
+                        umount_tree(mnt, 1, &umount_list);
                 retval = 0;
         }
         spin_unlock(&vfsmount_lock);
@@ -651,7 +671,7 @@ Enomem:
         if (res) {
                 LIST_HEAD(umount_list);
                 spin_lock(&vfsmount_lock);
-                umount_tree(res, &umount_list);
+                umount_tree(res, 0, &umount_list);
                 spin_unlock(&vfsmount_lock);
                 release_mounts(&umount_list);
         }
@@ -827,7 +847,7 @@ static int do_loopback(struct nameidata *nd, char *old_name, int recurse)
         if (err) {
                 LIST_HEAD(umount_list);
                 spin_lock(&vfsmount_lock);
-                umount_tree(mnt, &umount_list);
+                umount_tree(mnt, 0, &umount_list);
                 spin_unlock(&vfsmount_lock);
                 release_mounts(&umount_list);
         }
@@ -1023,12 +1043,12 @@ static void expire_mount(struct vfsmount *mnt, struct list_head *mounts,
          * Check that it is still dead: the count should now be 2 - as
          * contributed by the vfsmount parent and the mntget above
          */
-        if (atomic_read(&mnt->mnt_count) == 2) {
+        if (!propagate_mount_busy(mnt, 2)) {
                 /* delete from the namespace */
                 touch_namespace(mnt->mnt_namespace);
                 list_del_init(&mnt->mnt_list);
                 mnt->mnt_namespace = NULL;
-                umount_tree(mnt, umounts);
+                umount_tree(mnt, 1, umounts);
                 spin_unlock(&vfsmount_lock);
         } else {
                 /*
@@ -1647,7 +1667,7 @@ void __put_namespace(struct namespace *namespace)
         spin_unlock(&vfsmount_lock);
         down_write(&namespace_sem);
         spin_lock(&vfsmount_lock);
-        umount_tree(root, &umount_list);
+        umount_tree(root, 0, &umount_list);
         spin_unlock(&vfsmount_lock);
         up_write(&namespace_sem);
         release_mounts(&umount_list);
diff --git a/fs/pnode.c b/fs/pnode.c
index 2d572b88e6f6..7bc942d047cd 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -99,9 +99,94 @@ out:
         while (!list_empty(&tmp_list)) {
                 child = list_entry(tmp_list.next, struct vfsmount, mnt_hash);
                 list_del_init(&child->mnt_hash);
-                umount_tree(child, &umount_list);
+                umount_tree(child, 0, &umount_list);
         }
         spin_unlock(&vfsmount_lock);
         release_mounts(&umount_list);
         return ret;
 }
+
+/*
+ * return true if the refcount is greater than count
+ */
+static inline int do_refcount_check(struct vfsmount *mnt, int count)
+{
+        int mycount = atomic_read(&mnt->mnt_count);
+        return (mycount > count);
+}
+
+/*
+ * check if the mount 'mnt' can be unmounted successfully.
+ * @mnt: the mount to be checked for unmount
+ * NOTE: unmounting 'mnt' would naturally propagate to all
+ * other mounts its parent propagates to.
+ * Check if any of these mounts that **do not have submounts**
+ * have more references than 'refcnt'. If so return busy.
+ */
+int propagate_mount_busy(struct vfsmount *mnt, int refcnt)
+{
+        struct vfsmount *m, *child;
+        struct vfsmount *parent = mnt->mnt_parent;
+        int ret = 0;
+
+        if (mnt == parent)
+                return do_refcount_check(mnt, refcnt);
+
+        /*
+         * quickly check if the current mount can be unmounted.
+         * If not, we don't have to go checking for all other
+         * mounts
+         */
+        if (!list_empty(&mnt->mnt_mounts) || do_refcount_check(mnt, refcnt))
+                return 1;
+
+        for (m = propagation_next(parent, parent); m;
+                        m = propagation_next(m, parent)) {
+                child = __lookup_mnt(m, mnt->mnt_mountpoint, 0);
+                if (child && list_empty(&child->mnt_mounts) &&
+                    (ret = do_refcount_check(child, 1)))
+                        break;
+        }
+        return ret;
+}
+
+/*
+ * NOTE: unmounting 'mnt' naturally propagates to all other mounts its
+ * parent propagates to.
+ */
+static void __propagate_umount(struct vfsmount *mnt)
+{
+        struct vfsmount *parent = mnt->mnt_parent;
+        struct vfsmount *m;
+
+        BUG_ON(parent == mnt);
+
+        for (m = propagation_next(parent, parent); m;
+                        m = propagation_next(m, parent)) {
+
+                struct vfsmount *child = __lookup_mnt(m,
+                                        mnt->mnt_mountpoint, 0);
+                /*
+                 * umount the child only if the child has no
+                 * other children
+                 */
+                if (child && list_empty(&child->mnt_mounts)) {
+                        list_del(&child->mnt_hash);
+                        list_add_tail(&child->mnt_hash, &mnt->mnt_hash);
+                }
+        }
+}
+
+/*
+ * collect all mounts that receive propagation from the mount in @list,
+ * and return these additional mounts in the same list.
+ * @list: the list of mounts to be unmounted.
+ */
+int propagate_umount(struct list_head *list)
+{
+        struct vfsmount *mnt;
+
+        list_for_each_entry(mnt, list, mnt_hash)
+                __propagate_umount(mnt);
+        return 0;
+}
diff --git a/fs/pnode.h b/fs/pnode.h
index c62c72fad212..9b88ba06794a 100644
--- a/fs/pnode.h
+++ b/fs/pnode.h
@@ -29,4 +29,6 @@ static inline void set_mnt_shared(struct vfsmount *mnt)
 void change_mnt_propagation(struct vfsmount *, int);
 int propagate_mnt(struct vfsmount *, struct dentry *, struct vfsmount *,
                 struct list_head *);
+int propagate_umount(struct list_head *);
+int propagate_mount_busy(struct vfsmount *, int);
 #endif /* _LINUX_PNODE_H */