Re: [PATCH] Btrfs: fix race condition between writting and scrubing supers

[Stefan Behrens <sbehrens@giantdisaster.de>]

On 10/19/2013 06:17, Wang Shilong wrote:
> From: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
>
> Scrubing supers is not in a transaction context, when trying to
> write supers to disk, we should check if we are trying to
> scrub supers.Fix it.
>
> Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
> ---
>   fs/btrfs/disk-io.c     | 2 ++
>   fs/btrfs/transaction.c | 2 ++
>   2 files changed, 4 insertions(+)
>
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 419968e..0debb19 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -3582,7 +3582,9 @@ int btrfs_commit_super(struct btrfs_root *root)
>   		return ret;
>   	}
>
> +	btrfs_scrub_pause_super(root);
>   	ret = write_ctree_super(NULL, root, 0);
> +	btrfs_scrub_continue_super(root);
>   	return ret;
>   }
>
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index 277fe81..3ebcbbd 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -1892,7 +1892,9 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
>   		goto cleanup_transaction;
>   	}
>
> +	btrfs_scrub_pause_super(root);
>   	ret = write_ctree_super(trans, root, 0);
> +	btrfs_scrub_continue_super(root);
>   	if (ret) {
>   		mutex_unlock(&root->fs_info->tree_log_mutex);
>   		goto cleanup_transaction;
>

What kind of race do you see between writing the 4K superblock and scrub 
checking its checksum? Or in other words, what could happen?