Re: [PATCH v5 8/9] btrfs: Use sb_want_write() to protect label change.

[David Sterba <dsterba@suse.cz>]

On Fri, Jan 30, 2015 at 05:20:53PM +0800, Qu Wenruo wrote:
> Use the new vfs API sb_want_write() to do the write protection of the
> label change transaction.
> 
> Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
> ---
> v4:
>   Newly introduced.
> v5:
>   Change to use sb_want_write().
> ---
>  fs/btrfs/sysfs.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
> index b2e7bb4..3218245 100644
> --- a/fs/btrfs/sysfs.c
> +++ b/fs/btrfs/sysfs.c
> @@ -25,6 +25,7 @@
>  #include <linux/bug.h>
>  #include <linux/genhd.h>
>  #include <linux/debugfs.h>
> +#include <linux/mount.h>
>  
>  #include "ctree.h"
>  #include "disk-io.h"
> @@ -377,8 +378,6 @@ static ssize_t btrfs_label_store(struct kobject *kobj,
>  	int ret;
>  	size_t p_len;
>  
> -	if (fs_info->sb->s_flags & MS_RDONLY)
> -		return -EROFS;
>  
>  	/*
>  	 * p_len is the len until the first occurrence of either
> @@ -389,9 +388,15 @@ static ssize_t btrfs_label_store(struct kobject *kobj,
>  	if (p_len >= BTRFS_LABEL_SIZE)
>  		return -EINVAL;
>  
> +	ret = sb_want_write(fs_info->sb);
> +	if (ret)
> +		return ret;

Please move the check to the beginning of the function, where the
original MS_RDONLY check was.

> +
>  	trans = btrfs_start_transaction(root, 0);
> -	if (IS_ERR(trans))
> -		return PTR_ERR(trans);
> +	if (IS_ERR(trans)) {
> +		ret = PTR_ERR(trans);
> +		goto out;
> +	}
>  
>  	spin_lock(&root->fs_info->super_lock);
>  	memset(fs_info->super_copy->label, 0, BTRFS_LABEL_SIZE);
> @@ -399,9 +404,10 @@ static ssize_t btrfs_label_store(struct kobject *kobj,
>  	spin_unlock(&root->fs_info->super_lock);
>  	ret = btrfs_commit_transaction(trans, root);
>  
> +out:
> +	sb_drop_write(fs_info->sb);
>  	if (!ret)
>  		return len;
> -
>  	return ret;
>  }
>  BTRFS_ATTR_RW(label, btrfs_label_show, btrfs_label_store);