vfs: Keep a list of mounts on a mount point

To spot any possible problems call BUG if a mountpoint
is put when it's list of mounts is not empty.

AV: use hlist instead of list_head
Reviewed-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Eric W. Biederman <ebiederman@twitter.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/mount.h

@@ -21,6 +21,7 @@ struct mnt_pcp {
 struct mountpoint {
 	struct hlist_node m_hash;
 	struct dentry *m_dentry;
+	struct hlist_head m_list;
 	int m_count;
 };
 
@@ -51,6 +52,7 @@ struct mount {
 	struct mount *mnt_master;	/* slave is on master->mnt_slave_list */
 	struct mnt_namespace *mnt_ns;	/* containing namespace */
 	struct mountpoint *mnt_mp;	/* where is it mounted */
+	struct hlist_node mnt_mp_list;	/* list mounts with the same mountpoint */
 #ifdef CONFIG_FSNOTIFY
 	struct hlist_head mnt_fsnotify_marks;
 	__u32 mnt_fsnotify_mask;

fs/namespace.c

@@ -225,6 +225,7 @@ static struct mount *alloc_vfsmnt(const char *name)
 		INIT_LIST_HEAD(&mnt->mnt_share);
 		INIT_LIST_HEAD(&mnt->mnt_slave_list);
 		INIT_LIST_HEAD(&mnt->mnt_slave);
+		INIT_HLIST_NODE(&mnt->mnt_mp_list);
 #ifdef CONFIG_FSNOTIFY
 		INIT_HLIST_HEAD(&mnt->mnt_fsnotify_marks);
 #endif
@@ -731,6 +732,7 @@ static struct mountpoint *new_mountpoint(struct dentry *dentry)
 	mp->m_dentry = dentry;
 	mp->m_count = 1;
 	hlist_add_head(&mp->m_hash, chain);
+	INIT_HLIST_HEAD(&mp->m_list);
 	return mp;
 }
 
@@ -738,6 +740,7 @@ static void put_mountpoint(struct mountpoint *mp)
 {
 	if (!--mp->m_count) {
 		struct dentry *dentry = mp->m_dentry;
+		BUG_ON(!hlist_empty(&mp->m_list));
 		spin_lock(&dentry->d_lock);
 		dentry->d_flags &= ~DCACHE_MOUNTED;
 		spin_unlock(&dentry->d_lock);
@@ -784,6 +787,7 @@ static void detach_mnt(struct mount *mnt, struct path *old_path)
 	mnt->mnt_mountpoint = mnt->mnt.mnt_root;
 	list_del_init(&mnt->mnt_child);
 	hlist_del_init_rcu(&mnt->mnt_hash);
+	hlist_del_init(&mnt->mnt_mp_list);
 	put_mountpoint(mnt->mnt_mp);
 	mnt->mnt_mp = NULL;
 }
@@ -800,6 +804,7 @@ void mnt_set_mountpoint(struct mount *mnt,
 	child_mnt->mnt_mountpoint = dget(mp->m_dentry);
 	child_mnt->mnt_parent = mnt;
 	child_mnt->mnt_mp = mp;
+	hlist_add_head(&child_mnt->mnt_mp_list, &mp->m_list);
 }
 
 /*
@@ -1342,6 +1347,7 @@ void umount_tree(struct mount *mnt, int how)
 		if (how < 2)
 			p->mnt.mnt_flags |= MNT_SYNC_UMOUNT;
 		if (mnt_has_parent(p)) {
+			hlist_del_init(&p->mnt_mp_list);
 			put_mountpoint(p->mnt_mp);
 			mnt_add_count(p->mnt_parent, -1);
 			/* move the reference to mountpoint into ->mnt_ex_mountpoint */