linux-kernel - Re: [PATCH v2 1/4] dcache: Don't take unnecessary lock in d

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Date:	Fri, 5 Apr 2013 18:12:20 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Jeff Layton <jlayton@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>, Ian Kent <raven@...maw.net>,
	Sage Weil <sage@...tank.com>, Steve French <sfrench@...ba.org>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Eric Paris <eparis@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, autofs@...r.kernel.org,
	ceph-devel@...r.kernel.org, linux-cifs@...r.kernel.org,
	samba-technical@...ts.samba.org, linux-nfs@...r.kernel.org,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v2 1/4] dcache: Don't take unnecessary lock in d_count
 update

> @@ -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()?
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