Is RCU not safe enough to protect dcache hlist?

[Lee]

Hi, all:

I am studying the dcache in VFS recently . I found that hlist of dcache
is already protected by RCU in
  __d_lookup.Is it necessary  for the function -- d_lookup using
sequence lock to protect the hlist again?

struct dentry * d_lookup(struct dentry * parent, struct qstr * name)
{
     struct dentry * dentry = NULL;
     unsigned long seq;

         do {
                 seq = read_seqbegin(&rename_lock);
                 dentry = __d_lookup(parent, name);
                 if (dentry)
             break;
     } while (read_seqretry(&rename_lock, seq));
     return dentry;
}

Thanks
---
Rock Lee