On Mon, Dec 21, 2020 at 07:14:44PM +0800, Joseph Qi wrote:
> Hi Viro,
>
> On 12/21/20 2:26 PM, Al Viro wrote:
> > On Sun, Dec 20, 2020 at 08:09:27PM +0800, Liangyan wrote:
> >
> >> +++ b/fs/overlayfs/dir.c
> >> @@ -973,6 +973,7 @@ static char *ovl_get_redirect(struct dentry *dentry,
> >> bool abs_redirect)
> >> for (d = dget(dentry); !IS_ROOT(d);) {
> >> const char *name;
> >> int thislen;
> >> + struct dentry *parent = NULL;
> >>
> >> spin_lock(&d->d_lock);
> >> name = ovl_dentry_get_redirect(d);
> >> @@ -992,7 +993,22 @@ static char *ovl_get_redirect(struct dentry *dentry,
> >> bool abs_redirect)
> >>
> >> buflen -= thislen;
> >> memcpy(&buf[buflen], name, thislen);
> >> - tmp = dget_dlock(d->d_parent);
> >> + parent = d->d_parent;
> >> + if (unlikely(!spin_trylock(&parent->d_lock))) {
> >> + rcu_read_lock();
> >> + spin_unlock(&d->d_lock);
> >> +again:
> >> + parent = READ_ONCE(d->d_parent);
> >> + spin_lock(&parent->d_lock);
> >> + if (unlikely(parent != d->d_parent)) {
> >> + spin_unlock(&parent->d_lock);
> >> + goto again;
> >> + }
> >> + rcu_read_unlock();
> >> + spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED);
> >> + }
> >> + tmp = dget_dlock(parent);
> >> + spin_unlock(&parent->d_lock);
> >> spin_unlock(&d->d_lock);
> >
> > Yecchhhh.... What's wrong with just doing
> > spin_unlock(&d->d_lock);
> > parent = dget_parent(d);
> > dput(d);
> > d = parent;
> > instead of that?
> >
>
> Now race happens on non-RCU path in lookup_fast(), I'm afraid d_seq can
> not close the race window.
Explain, please. What exactly are you observing?
