On Mon, Apr 24, 2017 at 04:10:30PM +0300, Amir Goldstein wrote:
> On Mon, Apr 24, 2017 at 3:59 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> 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 ;-)
Hi Amir,
Thanks. Ok, so we are making sure if other cpu sees updated ->type, then
it is guaranteed that it isses updated ->upperdentry as well.
I feel it is worth to put a shortened version of explanation from miklos
in the comments. It will help to recall why did we put it. But it is just
me. May be it is obvious to others.
Vivek
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