[PATCH v2 1/4] dcache: Don't take unnecessary lock in d_count update

[Al Viro]

> @@ -635,22 +640,14 @@ struct dentry *dget_parent(struct dentry *dentry)
>  {
>  	struct dentry *ret;
>  
> -repeat:
> -	/*
> -	 * Don't need rcu_dereference because we re-check it was correct under
> -	 * the lock.
> -	 */
>  	rcu_read_lock();
> -	ret = dentry->d_parent;
> -	spin_lock(&ret->d_lock);
> -	if (unlikely(ret != dentry->d_parent)) {
> -		spin_unlock(&ret->d_lock);
> -		rcu_read_unlock();
> -		goto repeat;
> -	}
> +	ret = rcu_dereference(dentry->d_parent);
>  	rcu_read_unlock();
> +	if (dcount_inc_cmpxchg(ret))
> +		return ret;
> +	spin_lock(&ret->d_lock);

And WTF is going to protect your "ret" from being freed just as you'd done
rcu_read_unlock()?