Re: [PATCH v2 01/11] ovl: store path type in dentry

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Apr 24, 2017 at 12:14:06PM +0300, Amir Goldstein wrote:

[..]
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index c072a0c..671bac0 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -961,6 +961,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
>  	kfree(stack);
>  
>  	root_dentry->d_fsdata = oe;
> +	ovl_update_type(root_dentry, true);
>  
>  	realinode = d_inode(ovl_dentry_real(root_dentry));
>  	ovl_inode_init(d_inode(root_dentry), realinode, !!upperpath.dentry);
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index 1953986..6a857fb 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -70,21 +70,38 @@ bool ovl_dentry_weird(struct dentry *dentry)
>  enum ovl_path_type ovl_path_type(struct dentry *dentry)
>  {
>  	struct ovl_entry *oe = dentry->d_fsdata;
> -	enum ovl_path_type type = 0;
> +	enum ovl_path_type type = oe->__type;
>  
> -	if (oe->__upperdentry) {
> -		type = __OVL_PATH_UPPER;
> +	/* Matches smp_wmb() in ovl_update_type() */
> +	smp_rmb();
> +	return type;

Hi Amir,

I never manage to understand barriers so I will ask. Why this barrier is
required and what can go wrong if we don't use this barrier.

Vivek