Re: [PATCH v3 2/3] nfsd: Initial implementation of NFSv4 Courteous Server

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Sep 22, 2021 at 03:16:34PM -0700, dai.ngo@oracle.com wrote:
> 
> On 9/22/21 2:14 PM, J. Bruce Fields wrote:
> >On Thu, Sep 16, 2021 at 02:22:11PM -0400, Dai Ngo wrote:
> >>+/*
> >>+ * If the conflict happens due to a NFSv4 request then check for
> >>+ * courtesy client and set rq_conflict_client so that upper layer
> >>+ * can destroy the conflict client and retry the call.
> >>+ */
> >>+static bool
> >>+nfsd_check_courtesy_client(struct nfs4_delegation *dp)
> >>+{
> >>+	struct svc_rqst *rqst;
> >>+	struct nfs4_client *clp = dp->dl_recall.cb_clp;
> >>+	struct nfsd_net *nn = net_generic(clp->net, nfsd_net_id);
> >>+	bool ret = false;
> >>+
> >>+	if (!i_am_nfsd()) {
> >>+		if (test_bit(NFSD4_COURTESY_CLIENT, &clp->cl_flags)) {
> >>+			set_bit(NFSD4_DESTROY_COURTESY_CLIENT, &clp->cl_flags);
> >>+			mod_delayed_work(laundry_wq, &nn->laundromat_work, 0);
> >>+			return true;
> >>+		}
> >>+		return false;
> >>+	}
> >>+	rqst = kthread_data(current);
> >>+	if (rqst->rq_prog != NFS_PROGRAM || rqst->rq_vers < 4)
> >>+		return false;
> >>+	rqst->rq_conflict_client = NULL;
> >>+
> >>+	spin_lock(&nn->client_lock);
> >>+	if (test_bit(NFSD4_COURTESY_CLIENT, &clp->cl_flags) &&
> >>+				!mark_client_expired_locked(clp)) {
> >>+		rqst->rq_conflict_client = clp;
> >>+		ret = true;
> >>+	}
> >>+	spin_unlock(&nn->client_lock);
> >Check whether this is safe; I think the flc_lock may be taken inside of
> >this lock elsewhere, resulting in a potential deadlock?
> >
> >rqst doesn't need any locking as it's only being used by this thread, so
> >it's the client expiration stuff that's the problem, I guess.
> 
> mark_client_expired_locked needs to acquire cl_lock. I think the lock
> ordering is ok, client_lock -> cl_lock. nfsd4_exchange_id uses this
> lock ordering.

It's flc_lock (see locks.c) that I'm worried about.  I've got a lockdep
warning here, taking a closer look....

nfsd4_release_lockowner takes clp->cl_lock and then fcl_lock.

Here we're taking fcl_lock and then client_lock.

As you say, elsewhere client_lock is taken and then cl_lock.

So that's the loop, I think.

--b.