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

David Howells
2010-03-29T19:02:34Z

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.

> > - 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