linux-kernel - Re: [PATCH] NFS: Fix RCU warnings in nfs_inode_return_delegation

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Message-ID: <14003.1270122314@redhat.com>
Date:	Thu, 01 Apr 2010 12:45:14 +0100
From:	David Howells <dhowells@...hat.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	dhowells@...hat.com, Eric Dumazet <eric.dumazet@...il.com>,
	Trond.Myklebust@...app.com, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] NFS: Fix RCU warnings in nfs_inode_return_delegation_noreclaim() [ver #2]

Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> > I think it is incorrectly used.  Given that the rcu_dereference() in:
> > 
> > 	if (rcu_dereference(nfsi->delegation) != NULL) {
> > 		spin_lock(&clp->cl_lock);
> > 		delegation = nfs_detach_delegation_locked(nfsi, NULL);
> > 		spin_unlock(&clp->cl_lock);
> > 		if (delegation != NULL)
> > 			nfs_do_return_delegation(inode, delegation, 0);
> > 	}
> 
> And nfs_detach_delegation_locked() rechecks nfsi->delegation() under
> the lock, so this is a legitimate use.
> 
> The pointer is not held constant, but any changes will be accounted
> for and handled correctly.  So I would argue that the pointer value is
> in fact protected by the recheck-under-lock algorithm used here.

A legitimate use of what?

David
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