Re: [RFC PATCH 1/3] Implement generic freeze feature

[Christoph Hellwig <hch@infradead.org>]

> +	if (test_and_set_bit(BD_FREEZE_OP, &bdev->bd_state))
> +		return ERR_PTR(-EBUSY);
> +
> +	sb = get_super_without_lock(bdev);
> +
> +	/* If super_block has been already frozen, return. */
> +	if (sb && sb->s_frozen != SB_UNFROZEN) {
> +		put_super(sb);
> +		clear_bit(BD_FREEZE_OP, &bdev->bd_state);
> +		return sb;
> +	}

The BD_FREEZE_OP flag in the block_device already prevents multiple
freezes for a singe block device, so there is no need for this
additional check and the get_super_without_lock helper.

>  	down(&bdev->bd_mount_sem);

And with that flag bd_mount_sem is also obsolete for preventing the
multiple freeze operations.  We still need investigate what
synchronization we need vs unmount which also takes bd_mount_sem without
every having document what it exactly protects.