On Tue, Mar 16, 2010 at 02:43:15PM -0400, Trond Myklebust wrote:
> On Tue, 2010-03-16 at 11:10 -0700, Paul E. McKenney wrote:
> > On Tue, Mar 16, 2010 at 01:35:54PM -0400, Trond Myklebust wrote:
> > > On Tue, 2010-03-16 at 10:17 -0700, Paul E. McKenney wrote:
> > > > On Tue, Mar 16, 2010 at 11:51:30AM +0000, David Howells wrote:
> > > > > diff --git a/fs/nfs/delegation.c b/fs/nfs/delegation.c
> > > > > index 2563beb..a77c735 100644
> > > > > --- a/fs/nfs/delegation.c
> > > > > +++ b/fs/nfs/delegation.c
> > > > > @@ -37,8 +37,10 @@ static void nfs_free_delegation(struct nfs_delegation *delegation)
> > > > > {
> > > > > struct rpc_cred *cred;
> > > > >
> > > > > + rcu_read_lock();
> > > > > cred = rcu_dereference(delegation->cred);
> > > > > rcu_assign_pointer(delegation->cred, NULL);
> > > >
> > > > The lock is probably held here, in which case something like the
> > > > following would work well without needing the artificial rcu_read_lock()
> > > > and rcu_read_unlock():
> > >
> > > No. The lock is not held here. At this point, the delegation has been
> > > detached from the inode that pointed to it, and so we can free up its
> > > contents.
> >
> > OK. Is there some reference counter or pointer that can be checked to
> > verify that this data structure really is in a state that prevents
> > RCU readers from finding it?
>
> Yes. The RCU readers are supposed to grab the delegation->lock and then
> check the contents of the delegation->inode.
So would something like the following work, then?
cred = rcu_dereference_check(delegation->cred,
delegation->inode == NULL);
Or would some other check condition be more appropriate?
Thanx, Paul
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