Re: [PATCH] NFS: Fix RCU warnings in nfs_inode_return_delegation_noreclaim() [ver #2]

From: David Howells
Date: Tue Apr 06 2010 - 12:14:21 EST

Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> 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

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()?

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