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

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Date:	Mon, 29 Mar 2010 14:05:20 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Howells <dhowells@...hat.com>
Cc:	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]

On Mon, Mar 29, 2010 at 09:15:06PM +0100, David Howells wrote:
> 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.

But for !CONFIG_PROVE_RCU, rcu_dereference_check() is compiled out:

	#define rcu_dereference_check(p, c)     rcu_dereference_raw(p)

And rcu_dereference_raw() is the same as the old rcu_dereference().

So this should not be a problem, given that CONFIG_PROVE_RCU should not
be used for production systems.

							Thanx, Paul
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