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?