nfsd: provide callbacks on svc_xprt deletion

NFSv4.1 needs warning when a client tcp connection goes down, if that
connection is being used as a backchannel, so that it can warn the
client that it has lost the backchannel connection.
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

include/linux/sunrpc/svc_xprt.h

@@ -33,6 +33,16 @@ struct svc_xprt_class {
 	u32			xcl_max_payload;
 };
 
+/*
+ * This is embedded in an object that wants a callback before deleting
+ * an xprt; intended for use by NFSv4.1, which needs to know when a
+ * client's tcp connection (and hence possibly a backchannel) goes away.
+ */
+struct svc_xpt_user {
+	struct list_head list;
+	void (*callback)(struct svc_xpt_user *);
+};
+
 struct svc_xprt {
 	struct svc_xprt_class	*xpt_class;
 	struct svc_xprt_ops	*xpt_ops;
@@ -67,10 +77,25 @@ struct svc_xprt {
 	struct sockaddr_storage	xpt_remote;	/* remote peer's address */
 	size_t			xpt_remotelen;	/* length of address */
 	struct rpc_wait_queue	xpt_bc_pending;	/* backchannel wait queue */
+	struct list_head	xpt_users;	/* callbacks on free */
 
 	struct net		*xpt_net;
 };
 
+static inline void register_xpt_user(struct svc_xprt *xpt, struct svc_xpt_user *u)
+{
+	spin_lock(&xpt->xpt_lock);
+	list_add(&u->list, &xpt->xpt_users);
+	spin_unlock(&xpt->xpt_lock);
+}
+
+static inline void unregister_xpt_user(struct svc_xprt *xpt, struct svc_xpt_user *u)
+{
+	spin_lock(&xpt->xpt_lock);
+	list_del_init(&u->list);
+	spin_unlock(&xpt->xpt_lock);
+}
+
 int	svc_reg_xprt_class(struct svc_xprt_class *);
 void	svc_unreg_xprt_class(struct svc_xprt_class *);
 void	svc_xprt_init(struct svc_xprt_class *, struct svc_xprt *,

net/sunrpc/svc_xprt.c

@@ -156,6 +156,7 @@ void svc_xprt_init(struct svc_xprt_class *xcl, struct svc_xprt *xprt,
 	INIT_LIST_HEAD(&xprt->xpt_list);
 	INIT_LIST_HEAD(&xprt->xpt_ready);
 	INIT_LIST_HEAD(&xprt->xpt_deferred);
+	INIT_LIST_HEAD(&xprt->xpt_users);
 	mutex_init(&xprt->xpt_mutex);
 	spin_lock_init(&xprt->xpt_lock);
 	set_bit(XPT_BUSY, &xprt->xpt_flags);
@@ -881,6 +882,19 @@ static void svc_age_temp_xprts(unsigned long closure)
 	mod_timer(&serv->sv_temptimer, jiffies + svc_conn_age_period * HZ);
 }
 
+static void call_xpt_users(struct svc_xprt *xprt)
+{
+	struct svc_xpt_user *u;
+
+	spin_lock(&xprt->xpt_lock);
+	while (!list_empty(&xprt->xpt_users)) {
+		u = list_first_entry(&xprt->xpt_users, struct svc_xpt_user, list);
+		list_del(&u->list);
+		u->callback(u);
+	}
+	spin_unlock(&xprt->xpt_lock);
+}
+
 /*
  * Remove a dead transport
  */
@@ -913,6 +927,7 @@ void svc_delete_xprt(struct svc_xprt *xprt)
 	while ((dr = svc_deferred_dequeue(xprt)) != NULL)
 		kfree(dr);
 
+	call_xpt_users(xprt);
 	svc_xprt_put(xprt);
 }