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

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Date:	Fri, 05 Apr 2013 16:26:27 -0400
From:	Waiman Long <Waiman.Long@...com>
To:	Al Viro <viro@...IV.linux.org.uk>
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

On 04/05/2013 01:12 PM, Al Viro wrote:
>> @@ -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()?

I think I had made a mistake here. I should move the rcu_read_unlock() 
down to before the return statement as well as after the spin_lock(). 
Thank for pointing this out. I will fix that in the next version. 
Anything else that needs to be fixed?

Regards,
Longman
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