NFS: Allow walking nfs_client.cl_superblocks list outside client.c

We're about to move some fields from struct nfs_client to struct
nfs_server.  There is a many-to-one relationship between nfs_servers
and nfs_clients.  After these fields are moved to the nfs_server
struct, to visit all of the data in these fields that is owned by one
nfs_client, code will need to visit each nfs_server on the
cl_superblocks list for that nfs_client.

To serialize changes to the cl_superblocks list during these little
expeditions, protect the list with RCU.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/fs/nfs/nfs4renewd.c b/fs/nfs/nfs4renewd.c
index 72b6c580af13..cde5650ee5a2 100644
--- a/fs/nfs/nfs4renewd.c
+++ b/fs/nfs/nfs4renewd.c
@@ -63,9 +63,14 @@ nfs4_renew_state(struct work_struct *work)
 
         ops = clp->cl_mvops->state_renewal_ops;
         dprintk("%s: start\n", __func__);
-        /* Are there any active superblocks? */
-        if (list_empty(&clp->cl_superblocks))
+
+        rcu_read_lock();
+        if (list_empty(&clp->cl_superblocks)) {
+                rcu_read_unlock();
                 goto out;
+        }
+        rcu_read_unlock();
+
         spin_lock(&clp->cl_lock);
         lease = clp->cl_lease_time;
         last = clp->cl_last_renewal;