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

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Message-ID: <23274.1269893706@redhat.com>
Date:	Mon, 29 Mar 2010 21:15:06 +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:

> > > 	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.
> 
> Because Arnd Bergmann is working on a set of patches that makes sparse
> complain if you access an RCU-protected pointer directly, without using
> some flavor of rcu_dereference().
> 
> So your approach would work for the moment, but would need another
> change, probably in the 2.6.35 timeframe.

My objection to using rcu_dereference_check() here is that it's a dynamic
check: the compiler emits code to do it, since the lock/unlock status of what
the pointer points to cannot be determined easily at compiler time - and then
the barrier is interpolated anyway unnecessarily.

David
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