On Thu, Jan 23, 2020 at 09:16:01PM -0800, Eric Biggers wrote:
> On Fri, Jan 24, 2020 at 01:04:25PM +0800, Gao Xiang wrote:
> > > diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
> > > index 8964778aabefb..0129d14629881 100644
> > > --- a/fs/ext4/dir.c
> > > +++ b/fs/ext4/dir.c
> > > @@ -671,9 +671,11 @@ static int ext4_d_compare(const struct dentry *dentry, unsigned int len,
> > > const char *str, const struct qstr *name)
> > > {
> > > struct qstr qstr = {.name = str, .len = len };
> > > - struct inode *inode = dentry->d_parent->d_inode;
> > > + const struct dentry *parent = READ_ONCE(dentry->d_parent);
> >
> > I'm not sure if we really need READ_ONCE d_parent here (p.s. d_parent
> > won't be NULL anyway), and d_seq will guard all its validity. If I'm
> > wrong, correct me kindly...
> >
> > Otherwise, it looks good to me...
> > Reviewed-by: Gao Xiang <gaoxiang25@xxxxxxxxxx>
> >
>
> While d_parent can't be set to NULL, it can still be changed concurrently.
> So we need READ_ONCE() to ensure that a consistent value is used.
If I understand correctly, unlazy RCU->ref-walk will be guarded by
seqlock, and for ref-walk we have d_lock (and even parent lock)
in relative paths. So I prematurely think no race of renaming or
unlinking evenually.
I'm curious about that if experts could correct me about this.
Thanks,
Gao Xiang
>
> - Eric
