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

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Date:	Tue, 16 Mar 2010 12:15:29 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Trond Myklebust <Trond.Myklebust@...app.com>
Cc:	David Howells <dhowells@...hat.com>, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] NFS: Fix RCU warnings in
	nfs_inode_return_delegation_noreclaim()

On Tue, Mar 16, 2010 at 02:43:15PM -0400, Trond Myklebust wrote:
> On Tue, 2010-03-16 at 11:10 -0700, Paul E. McKenney wrote: 
> > On Tue, Mar 16, 2010 at 01:35:54PM -0400, Trond Myklebust wrote:
> > > On Tue, 2010-03-16 at 10:17 -0700, Paul E. McKenney wrote: 
> > > > On Tue, Mar 16, 2010 at 11:51:30AM +0000, David Howells wrote:
> > > > > diff --git a/fs/nfs/delegation.c b/fs/nfs/delegation.c
> > > > > index 2563beb..a77c735 100644
> > > > > --- a/fs/nfs/delegation.c
> > > > > +++ b/fs/nfs/delegation.c
> > > > > @@ -37,8 +37,10 @@ static void nfs_free_delegation(struct nfs_delegation *delegation)
> > > > >  {
> > > > >  	struct rpc_cred *cred;
> > > > > 
> > > > > +	rcu_read_lock();
> > > > >  	cred = rcu_dereference(delegation->cred);
> > > > >  	rcu_assign_pointer(delegation->cred, NULL);
> > > > 
> > > > The lock is probably held here, in which case something like the
> > > > following would work well without needing the artificial rcu_read_lock()
> > > > and rcu_read_unlock():
> > > 
> > > No. The lock is not held here. At this point, the delegation has been
> > > detached from the inode that pointed to it, and so we can free up its
> > > contents.
> > 
> > OK.  Is there some reference counter or pointer that can be checked to
> > verify that this data structure really is in a state that prevents
> > RCU readers from finding it?
> 
> Yes. The RCU readers are supposed to grab the delegation->lock and then
> check the contents of the delegation->inode.

So would something like the following work, then?

	cred = rcu_dereference_check(delegation->cred,
				     delegation->inode == NULL);

Or would some other check condition be more appropriate?

							Thanx, Paul
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