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

Paul E. McKenney
2010-03-29T19:22:02Z

On Mon, Mar 29, 2010 at 08:02:28PM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@linux.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.

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.

> > > - 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.

I must defer to Trond on this one.

Thanx, Paul