On Mon, Aug 18, 2008 at 09:28:19PM +0900, Takashi Sato wrote:

+	down(&bdev->bd_freeze_sem);
+	bdev->bd_freeze_count++;
+	if (bdev->bd_freeze_count > 1) {
+		sb = get_super(bdev);
+		drop_super(sb);
+		up(&bdev->bd_freeze_sem);
+		return sb;
+	}
+
 	down(&bdev->bd_mount_sem);

Now you have a reference counter of freezes which actually is pretty
sensible, but also needs some documentation.  What I don't understand
here at all is why you do the get_super/drop_super in the already frozen
case.

Now that the freeze_count has replaced one of the uses of bd_mount_sem
you should also replace the other use in the unmount path by simply
checking for the freez_count and abort if it's set.  To do so you'll
need to hold the bd_mount_sem over the whole unmount operation to
prevent new frezes from coming in.

As others noted it should be a mutex and not a semaphore.

 /*
+ * ioctl_freeze - Freeze the filesystem.
+ *
+ * @filp:	target file
+ *
+ * Call freeze_bdev() to freeze the filesystem.
+ */
+static int ioctl_freeze(struct file *filp)

This is not quite kerneldcoc format, which would ne a /** as commnt
start.  But I don't think the comment is actually needed, it's a pretty
obvious file scope function. (Same commnt also applies to ioctl_thaw)

+	struct super_block *sb = filp->f_path.dentry->d_inode->i_sb;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	/* If filesystem doesn't support freeze feature, return. */
+	if (sb->s_op->write_super_lockfs == NULL)
+		return -EOPNOTSUPP;
+
+	/* If a regular file or a directory isn't specified, return. */
+	if (sb->s_bdev == NULL)
+		return -EINVAL;

I don't understand this commnt.  What you are checking is that the
filesystem has a non-NULL s_bdev, which implies a not blockdevice-backed
filesystem.