[PATCH v2] nfs: fix a deadlock in nfs client initialization

[Scott Mayhew <smayhew@redhat.com>]

The following deadlock can occur between a process waiting for a client
to initialize in while walking the client list during nfsv4 server trunking
detection and another process waiting for the nfs_clid_init_mutex so it
can initialize that client:

Process 1                               Process 2
---------                               ---------
spin_lock(&nn->nfs_client_lock);
list_add_tail(&CLIENTA->cl_share_link,
        &nn->nfs_client_list);
spin_unlock(&nn->nfs_client_lock);
                                        spin_lock(&nn->nfs_client_lock);
                                        list_add_tail(&CLIENTB->cl_share_link,
                                                &nn->nfs_client_list);
                                        spin_unlock(&nn->nfs_client_lock);
                                        mutex_lock(&nfs_clid_init_mutex);
                                        nfs41_walk_client_list(clp, result, cred);
                                        nfs_wait_client_init_complete(CLIENTA);
(waiting for nfs_clid_init_mutex)

Make sure nfs_match_client() only evaluates clients that have completed
initialization in order to prevent that deadlock.

Signed-off-by: Scott Mayhew <smayhew@redhat.com>
---
 fs/nfs/client.c     | 11 +++++++++++
 fs/nfs/nfs4client.c |  9 ++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/fs/nfs/client.c b/fs/nfs/client.c
index 0ac2fb1..7e42380 100644
--- a/fs/nfs/client.c
+++ b/fs/nfs/client.c
@@ -291,12 +291,23 @@ static struct nfs_client *nfs_match_client(const struct nfs_client_initdata *dat
 	const struct sockaddr *sap = data->addr;
 	struct nfs_net *nn = net_generic(data->net, nfs_net_id);
 
+again:
 	list_for_each_entry(clp, &nn->nfs_client_list, cl_share_link) {
 	        const struct sockaddr *clap = (struct sockaddr *)&clp->cl_addr;
 		/* Don't match clients that failed to initialise properly */
 		if (clp->cl_cons_state < 0)
 			continue;
 
+		/* If a client is still initializing then we need to wait */
+		if (clp->cl_cons_state > NFS_CS_READY) {
+			spin_unlock(&nn->nfs_client_lock);
+			refcount_inc(&clp->cl_count);
+			nfs_wait_client_init_complete(clp);
+			nfs_put_client(clp);
+			spin_lock(&nn->nfs_client_lock);
+			goto again;
+		}
+
 		/* Different NFS versions cannot share the same nfs_client */
 		if (clp->rpc_ops != data->nfs_mod->rpc_ops)
 			continue;
diff --git a/fs/nfs/nfs4client.c b/fs/nfs/nfs4client.c
index 12bbab0..39dd39c 100644
--- a/fs/nfs/nfs4client.c
+++ b/fs/nfs/nfs4client.c
@@ -411,8 +411,15 @@ struct nfs_client *nfs4_init_client(struct nfs_client *clp,
 	if (error < 0)
 		goto error;
 
-	if (clp != old)
+	if (clp != old) {
 		clp->cl_preserve_clid = true;
+		/*
+		 * Mark the client as having failed initialization so other
+		 * processes walking the nfs_client_list in nfs_match_client()
+		 * won't try to use it.
+		 */
+		nfs_mark_client_ready(clp, -EPERM);
+	}
 	nfs_put_client(clp);
 	clear_bit(NFS_CS_TSM_POSSIBLE, &clp->cl_flags);
 	return old;
-- 
2.9.5