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

[Amir Goldstein <amir73il@gmail.com>]

On Mon, Apr 24, 2017 at 3:59 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
>
> 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.
>

Hi Vivek,

Miklos was kind enough to answer that question for me when he made
the comment about missing memmory barrier on v1 of the patch:
http://www.spinics.net/lists/linux-unionfs/msg01687.html

Whether or not I got it right, we shall see shortly ;-)

Amir.