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

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Message-ID: <19192.1269889348@redhat.com>
Date:	Mon, 29 Mar 2010 20:02:28 +0100
From:	David Howells <dhowells@...hat.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	dhowells@...hat.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:

> >  	spin_lock(&clp->cl_lock);
> > -	if (rcu_dereference(nfsi->delegation) != NULL) {
> > +	if (nfsi->delegation != NULL) {
> 
> And this one.  I thought that Trond said that clp->cl_lock protects
> this one, in which case this should work:
> 
> 	if (rcu_dereference_check(nfsi->delegation,
> 				  lockdep_is_held(&clp->cl_lock)) != NULL) {

If clp->cl_lock protects this pointer, why the need for rcu_dereference_check()
at all?  The check is redundant since the line above gets the very lock we're
checking for.

> > -	if (rcu_dereference(nfsi->delegation) != NULL) {
> > +	if (nfsi->delegation != NULL) {
> 
> And this one, although the check for cp->cl_lock obviously won't work here.
> 
> >  		spin_lock(&clp->cl_lock);
> >  		delegation = nfs_detach_delegation_locked(nfsi, NULL);
> >  		spin_unlock(&clp->cl_lock);

On this one, why does nfsi->delegation need a memory barrier interpolating
afterwards?  It has an implicit one in the form of the spin_lock() immediately
after, if the value of the pointer wasn't NULL.  What two memory accesses is
the memory barrier ordering?

Ditto on the next one.

David
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