netdev - Re: [PATCH v3 24/25] sunrpc: Change how dentry's d

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Message-ID: <1373302428.2231.9.camel@leira.trondhjem.org>
Date:	Mon, 8 Jul 2013 16:53:52 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Waiman Long <Waiman.Long@...com>, Jeff Layton <jlayton@...hat.com>,
	"Miklos Szeredi" <mszeredi@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	"David S. Miller" <davem@...emloft.net>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH v3 24/25] sunrpc: Change how dentry's d_lock field is
 accessed

On Thu, 2013-07-04 at 05:20 +0100, Al Viro wrote:
> On Wed, Jul 03, 2013 at 04:25:32PM -0400, Waiman Long wrote:
> > There is no change in logic and everything should just work.
> 
> > -		spin_lock(&file->f_path.dentry->d_lock);
> > +		d_lock(file->f_path.dentry);
> >  		if (!d_unhashed(file->f_path.dentry))
> >  			clnt = RPC_I(inode)->private;
> >  		if (clnt != NULL && atomic_inc_not_zero(&clnt->cl_count)) {
> > -			spin_unlock(&file->f_path.dentry->d_lock);
> > +			d_unlock(file->f_path.dentry);
> 
> Could somebody explain WTF is being protected here?  It's not ->private -
> that gets set (and, more importantly, cleared) without ->d_lock in sight.
> Trond, that seems to be your code from about three years ago (introduced
> in "SUNRPC: Fix a race in rpc_info_open").  What's going on there?

AFAICR we're using the fact that the dentry will remain hashed until
we're in the process of freeing up the rpc_client. By testing that the
dentry is hashed under the dentry->d_lock, we are assured that the
non-NULL 'clnt' is still pointing to a valid rpc_client, and that it is
safe to access clnt->cl_count.

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com