[PATCH] count ghost references to vfsmounts

make propagate_mount_busy() exclude references from the vfsmounts
that had been isolated by umount_tree() and are just waiting for
release_mounts() to dispose of their ->mnt_parent/->mnt_mountpoint.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -548,6 +548,7 @@ void release_mounts(struct list_head *head)
 			m = mnt->mnt_parent;
 			mnt->mnt_mountpoint = mnt->mnt_root;
 			mnt->mnt_parent = mnt;
+			m->mnt_ghosts--;
 			spin_unlock(&vfsmount_lock);
 			dput(dentry);
 			mntput(m);
@@ -572,8 +573,10 @@ void umount_tree(struct vfsmount *mnt, int propagate, struct list_head *kill)
 		__touch_mnt_namespace(p->mnt_ns);
 		p->mnt_ns = NULL;
 		list_del_init(&p->mnt_child);
-		if (p->mnt_parent != p)
+		if (p->mnt_parent != p) {
+			p->mnt_parent->mnt_ghosts++;
 			p->mnt_mountpoint->d_mounted--;
+		}
 		change_mnt_propagation(p, MS_PRIVATE);
 	}
 }

fs/pnode.c

@@ -225,7 +225,7 @@ out:
  */
 static inline int do_refcount_check(struct vfsmount *mnt, int count)
 {
-	int mycount = atomic_read(&mnt->mnt_count);
+	int mycount = atomic_read(&mnt->mnt_count) - mnt->mnt_ghosts;
 	return (mycount > count);
 }
 

include/linux/mount.h

@@ -61,6 +61,7 @@ struct vfsmount {
 	atomic_t mnt_count;
 	int mnt_expiry_mark;		/* true if marked for expiry */
 	int mnt_pinned;
+	int mnt_ghosts;
 };
 
 static inline struct vfsmount *mntget(struct vfsmount *mnt)