Is RCU not safe enough to protect dcache hlist?

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

RCU is useful when you can tolerate a bit of latency in other users finding out about updated values and/or slightly stale values. That's not an option for d_lookup(), which has to check the most recent value in order to avoid race conditions.

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In __d_lookup(), one hlist is protected well with RCU. But this is not enough for the dentry_hashtable, so d_lookup() uses some mechanism to protect the dentry_hashtable, in this condition, the mechanism is sequence lock :-)