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

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Date:	Tue, 06 Apr 2010 17:14:03 +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:

> > > So you have objected to needless memory barriers.  How do you feel
> > > about possibly needless ACCESS_ONCE() calls?
> > 
> > That would work here since it shouldn't emit any excess instructions.
> 
> And here is the corresponding patch.  Seem reasonable?

Actually, now I've thought about it some more.  No, it's not reasonable.
You've written:

    This patch adds a variant of rcu_dereference() that handles situations
    where the RCU-protected data structure cannot change, perhaps due to
    our holding the update-side lock, or where the RCU-protected pointer is
    only to be tested, not dereferenced.

But if we hold the update-side lock, then why should we be forced to use
ACCESS_ONCE()?

In fact, if we don't hold the lock, but we want to test the pointer twice in
succession, why should we be required to use ACCESS_LOCK()?

David
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