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From: David Howells on 29 Mar 2010 15:10
Paul E. McKenney <paulmck(a)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.

> > - 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.

David
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From: David Howells on 29 Mar 2010 16:20
Paul E. McKenney <paulmck(a)linux.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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From: David Howells on 29 Mar 2010 18:30
Paul E. McKenney <paulmck(a)linux.vnet.ibm.com> wrote:

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

Exactly. That's the other half of the issue - that interpolates a redundant
memory barrier.

David
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From: David Howells on 29 Mar 2010 19:00
Paul E. McKenney <paulmck(a)linux.vnet.ibm.com> wrote:

> Only on Alpha. Otherwise only a volatile access.

Whilst that is true, it's the principle of the thing. The extra barrier
shouldn't be emitted on Alpha. If Alpha's no longer important, then can we
scrap smp_read_barrier_depends()?

My point is that some of these rcu_dereference*()'s are unnecessary. If
there're required for correctness tracking purposes, fine; but can we have a
macro that is just a dummy for the purpose of stripping the pointer Sparse
annotation? One that doesn't invoke rcu_dereference_raw() and interpolate a
barrier, pretend or otherwise, when there's no second reference to order
against.

David
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From: David Howells on 30 Mar 2010 12:40
Paul E. McKenney <paulmck(a)linux.vnet.ibm.com> wrote:

> Scrap this one -- Arnd has it covered, under the much better name
> of rcu_dereference_const().

Not convinced of that name either. That sounds like the RCU dereference of
constant (R/O) data.

David
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